[google compute] improved API coverage for Projects - redux

This PR replaces #399.

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`.