Added route for Admin API to support guest upgrade

Added optional metadata param to API

1. Changed route to {tenant_id}/mgmt/instances/{instance_id}/upgrade
2. Changed POST body to send properties inside an upgrade dict
3. Removed unimplemented method from guest agent manager
4. Swap out rpc call to use _cast
5. Removed instance_id from rpc call
6. Removed strategy from api calls
7. Switched out import of guestagent api in favor of common.remote
8. Rename UpgradeNotifier to UpgradeMessageSender

Change-Id: I12d09fd70ab028da1cd2818fe380310e99b66d2d
Implements: blueprint upgrade-guestagent
Closes-Bug: #1211924
Merge "Added route for Admin API to support guest upgrade"