[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`.
Those methods are located on a class which is already not a part of a standard Libcloud API so adding `ex_` prefix here is unnecessary and redundant. And other methods are located on the NodeDriver class which means they need to have `ex_` prefix.

As far as having methods on both - node driver and project class - it's something we do for the user convenience.