Fix up an old RpcProxy assumption

This patch fixes one last bit of code that assumed that the base
PluginApi used RpcProxy.  This code is now fixed to use the proper
oslo.messaging APIs instead.

Part of blueprint drop-rpc-compat.

Change-Id: I4adc321c07e6b5ad45a01f163b4c7e1ab74bcafd
Merge "Fix up an old RpcProxy assumption"