In commit ce4dbbd7, the BASE_RPC_API_VERSION was changed to '2.1' in
nova.compute.rpcapi. This should remain '2.0' until the API goes to
3.X. Only specify version '2.1' for the method that requires it
(rebuild_instance).
Add a NOTE above each instance of BASE_RPC_API_VERSION to try to help
clarify this for the future.
Change-Id: I06cef44c905e8966505d5fb5b1046d6a0c75d533