Drop RpcProxy usage from oneconvergence plugin

This patch removes usage of the RpcProxy compatibility class from the
oneconvergence plugin.  The equivalent usage of oslo.messaging APIs is
now used intead.

Part of blueprint drop-rpc-compat.

Change-Id: Ifa74849ba9e3dc09cbb8cb3a723aeaa2b25b87ad
Merge "Drop RpcProxy usage from oneconvergence plugin"