Metadata/credentials update, json key support

[Temikus]

Hello @tyler-ball, @someara,

Please review the following patch that starts my improvements of google-chef-provisioning, it contains the following changes:

• Cleaning up credential setup
  • Removing passphrase setting from keys to use the same logic other libs (e.g. fog) use.
  • Changing issuer to google_client_email for clarity and consistency with other libs.
• Fixed private key search logic to scan all :private_key_paths
• Fixed metadata setup, project items are no longer overwritten when setting up keys.
• Adding JSON key support, key validation logic (please, take a look at this especially, I want to be sure I'm idiomatic with Chef):
  • JSON keys are now supported
  • Key existence and setup is now validated
  • Google_client_email is extracted from JSON if none was specified
• Updating docs, .gitignore, rubocop files, locking deps.

Also, I have 1 additional question with this pull-request:

• Currently I'm raising strings (runtime errors) on validation, is that ok or is there some built-in handler that may be better? Or do we need to write our own?

/CC @erjohnso, @lykos, @mbrukman

[Temikus]

@Lykos - thanks for the comments, added an exception when no keys have been picked up

[tyler-ball]

LGTM!

[erjohnso]

Thanks @Temikus and thanks for merging @tyler-ball!