Adds integration tests for gce module.
Adds integration tests for gce_pd module.
Adds support for creating GCE persistent disks from images
Adds support for creating GCE persistent disks from snapshots
Adds support for attaching persistent disks to GCE instances
Changes cleanup_gce.py to use gce_credentials module.
Adds version_added property to new attributes in gce and gce_pd.
Merge pull request #6976 from clconway/gce-snapshots

Adds support for snapshots and extra persistent disks to the gce modules