Support service_accounts option in Fog

[jgoldschrafe]

Pass the service_accounts parameter to Fog, allowing instances to be created with an attached service account that has specific permissions on the project. For example, to create a test instance with user info, compute (read-only) and storage (read-only):

platforms:
  - name: ubuntu-12.04
    driver_config:
      # ...
      service_accounts:
        - userinfo.email
        - compute.readonly
        - devstorage.read_only

I haven't found a full list of OAuth scopes permitted in Google Compute Engine, but you can get a partial list by creating an instance through the web console and viewing the generated REST command.

[anl]

Merged - thank you!