Update cookbook structure to be more idiomatic

[tknerr]

This PR updates the structure of this repo to reflect the more idiomatic cookbook structure that would be also generated using chef generate cookbook .... (note that the earlier cookbook structure is valid too, but it's the one we found to be more optimal for developer VMs, but less optimal for cookbooks that represent server VMs)

So this PR:

• moves the contents from cookbooks/vm to top level and gets rid of the update-vm.sh script
• adds a Gemfile to lock the gem environment so we are not dependent on a specific version of ChefDK and so that we can update individual gems if desired (run bundle install to download / install them)
• adds a Rakefile to provide a common interface for running tasks and hiding the nitty gritty commandline parameters from the user (run rake -T to list available tasks):

$ rake -T
rake chefspec     # run chefspec examples
rake foodcritic   # run foodcritic lint checks
rake integration  # run test-kitchen integration tests
rake rubocop      # check Ruby code style with rubocop
rake unit         # run all unit-level tests

• adds a .rubocop.yml config and makes ruby level linting via rubocop pass (run rake rubocop)
• makes chef level linting via foodcritic pass (run rake foodcritic)
• updated the .kitchen.yml config to lock the Chef version, use chef zero, and make use of caching if vagrant-cachier plugin is installed. All tests are still passing. (run rake integration)
• updated the example Vagrantfile to use chef_zero + berkshelf

Still TODO:

• [x] update README
• [x] update (or remove?) Vagrantfile
• [x] add circelci build integration (will be added in a separate PR)

/cc @damphyr @jahnf

[tknerr]

@damphyr @a-zen @jahnf this PR is now complete and ready for review.

Please ignore the failing CircleCI build for now -- it's a docker related issue and all integration tests are passing when run in VirtualBox. I will fix this later in a separate PR

[a-zen]

Very well done thank you very much. Sorry for the delay with the feedback.

[tknerr]

:+1: thanks for merging!