Drop config changes.
Use osEnv prefix for env constants.
Add region and project-id to config.
cfgClientId -> cfgClientID.
Make client-email an immutable field.
Add auth and newConnection to environConfig.
Move GCE auth/connection validation over to gce.go.
Fix private-key validation.
Rename broker.go.
Add TODO comment.
Add TODO comment.
Drop environ.SupportsUnitAssignment method implementation.
Clean up getting instances and implement environ.Instances.
Add some doc comments.
Drop the (unused) auth field in environ.
provider/gce: move state policy methods to own file
provider/gce: add some initial instanceTypes
provider/gce: implemented PrecheckInstance