Refactored the client code and added tests

[Lykos]

• DRYed up the client code by moving some common stuff to the base class.
• Introduced our own exception class. It still needs to be fleshed out to properly parse API responses, though.
• Moved some logic that doesn't belong to the driver to the client code, e.g. the retry logic for API calls. There is still a bit more to be done on this point.
• Made the connection between our client code and the google API easier to understand by having one class for one API service. Before, there was the operations client which wrapped both the global_operations and the zone_operations service.
• Some of the client code returns relatively complicated hashes that are accessed in different places across the code. To make this more manageable, I created wrapper objects for those hashes, so the code to access the internals of those hashes is in one place. E.g. extracting an IP address for an instance from the hash returned by the API is a bit complicated, now the wrapper class Instance provides a method determine_remote_host that does this.
• Refactored the way the metadata API works. Before, each metadata change triggered two API calls. Now you do one read call in the beginning, then apply all the changes you want to do and then reupload at the end.
• Added unit tests for all the client classes.
• Added more method and class comments.
• Added "provides :google_key_pair" to GoogleKeyPair resource to resolve a warning.

[Temikus]

@Lykos See my small comment + I do have issues starting up if I rebase on your PR with the following repro, I'm digging into it:

temikus λ bundle exec chef-client --color -z test_cluster.rb
[2016-02-22T14:46:58-08:00] WARN: No config file found or specified on command line, using command line options.
[2016-02-22T14:46:58-08:00] WARN: No cookbooks directory found at or above current directory.  Assuming /Users/temikus/Code/chef-dev/chef-provisioning-google.
Starting Chef Client, version 12.5.1
resolving cookbooks for run list: []
Synchronizing Cookbooks:
Compiling Cookbooks...
[2016-02-22T14:47:03-08:00] WARN: Node a-macbook.contozo.com has an empty run list.
Converging 2 resources
Recipe: @recipe_files::/Users/temikus/Code/chef-dev/chef-provisioning-google/test_cluster.rb
  * google_key_pair[chef-provisioning] action create[2016-02-22T14:47:03-08:00] WARN: Class Chef::Provider::GoogleKeyPair does not declare 'provides :google_key_pair'.
[2016-02-22T14:47:03-08:00] WARN: This will no longer work in Chef 13: you must use 'provides' to use the resource's DSL.

    - adding key for username temikus
    - ensuring we store metadata mapping for google_key_pair[chef-provisioning]
    ================================================================================
    Error executing action `create` on resource 'google_key_pair[chef-provisioning]'
    ================================================================================

    NoMethodError
    -------------
    undefined method `projects_client' for #<Chef::Provisioning::GoogleDriver::Driver:0x007fe592163c60>

    Resource Declaration:
    ---------------------
    # In /Users/temikus/Dropbox/Code/chef-dev/chef-provisioning-google/test_cluster.rb

     11: google_key_pair "chef-provisioning" do
     12:   private_key_path 'google_nouveaux'
     13:   public_key_path 'google_nouveaux.pub'
     14:   allow_overwrite true
     15: end
     16:

    Compiled Resource:
    ------------------
    # Declared in /Users/temikus/Dropbox/Code/chef-dev/chef-provisioning-google/test_cluster.rb:11:in `from_file'

    google_key_pair("chef-provisioning") do
      action [:create]
      retries 0
      retry_delay 2
      default_guard_interpreter :default
      declared_type :google_key_pair
      cookbook_name "@recipe_files"
      recipe_name "/Users/temikus/Code/chef-dev/chef-provisioning-google/test_cluster.rb"
      private_key_path "/Users/temikus/.chef/keys/google_nouveaux"
      public_key_path "/Users/temikus/.chef/keys/google_nouveaux.pub"
      allow_overwrite true
    end

Running handlers:
[2016-02-22T14:47:05-08:00] ERROR: Running exception handlers
Running handlers complete
[2016-02-22T14:47:05-08:00] ERROR: Exception handlers complete
Chef Client failed. 0 resources updated in 07 seconds
[2016-02-22T14:47:05-08:00] FATAL: Stacktrace dumped to /Users/temikus/.chef/local-mode-cache/cache/chef-stacktrace.out
[2016-02-22T14:47:05-08:00] ERROR: google_key_pair[chef-provisioning] (@recipe_files::/Users/temikus/chef-dev/chef-provisioning-google/test_cluster.rb line 11) had an error: NoMethodError: undefined method `projects_client' for #<Chef::Provisioning::GoogleDriver::Driver:0x007fe592163c60>
[2016-02-22T14:47:05-08:00] FATAL: Chef::Exceptions::ChildConvergeError: Chef run process exited unsuccessfully (exit code 1)

With the following recipe:

require 'chef/provisioning/google_driver'

with_driver 'google:us-central1-a:dev-utility-AAAAA',
  :google_credentials => {
    :json_key_path => '/Users/temikus/.gce/graphite-sandbox-xxxxx.json',
  }

google_key_pair "chef-provisioning" do
  private_key_path 'google_nouveaux'
  public_key_path 'google_nouveaux.pub'
  allow_overwrite true
end

machine 'chef-provisioning-test-basic' do
  machine_options key_name: 'google_nouveaux'
  action [:converge, :destroy]
end

[Temikus]

@Lykos See my comments above, esp. the key creation issues, I think we need to make sure to test all scenarios - key doesn't exist and gets created, key exists, key exists but doesn't match, etc.

Otherwise - looks good so far :+1:

[Lykos]

Fixed the issues.

[Temikus]

@tyler-ball, @someara, can you please take a look?

/CC @erjohnso

[tas50]

Closing in favor of #8