Sync some of Instance* models with migrations

Update db.models description for:

 * InstanceFault
 * InstanceAction
 * InstanceActionEvent

That brings it in sync with the actual database
schemas.

Blueprint: db-sync-models-with-migrations

Change-Id: I5fae28a86064a0582fd2a53a1a7277947b6a3525
Merge "Sync some of Instance* models with migrations"