Convert several uses of RpcCallback

Convert several places in the code that were using the RpcCallback
compatibility class.  This class eased the transition to
oslo.messaging.  This patch drops the usage of this class in favor of
direct usage of the appropriate oslo.messaging API.

Part of blueprint drop-rpc-compat.

Change-Id: I955958e836635d43dabc7b39d050c1418b18cf8f
Merge "Convert several uses of RpcCallback"