[google|compute] Enable passing the google key as a String

Google::APIClient::KeyUtils.load_from_pkcs12 allows passing the contents
of the PKCS12 key as a String instead of the PKCS12 file location.

Adding a new initialize param (google_key_string) to enable the
automation on those environments where you can not pass a file.
Merge pull request #2879 from frodenas/gce_key

[google|compute] Enable passing the google_key_location as a String