[google compute] improved API coverage for Projects

This PR adds support for:
 * Setting common instance metadata. Common instance (or project-wide) metadata is typically used for defining a `startup-script` and setting `sshKeys`.
 * Setting a usage export bucket. The usage export feature allows storing compute engine resource usage to Google Cloud Storage.

For setting common instance metadata, there is a safety check (boolean flag) to that must be set to forcefully overwrite or remove `sshKeys`.
@erjohnso thanks for contributing tests, that's great. @Kami do you see an issue with the fixture file name using camel case. A detail really, but since we try to be consistent, maybe _ would be more pythonic even in file names ?