Drop RpcProxy usage from MetadataPluginApi

Drop the usage of the RpcProxy compatibility class from
MetadataPluginApi.  The equivalent direct usage of oslo.messaging APIs
are used instead.

Part of blueprint drop-rpc-compat.

Change-Id: I1764dd715237bc76dae2789121850f69b8f82888
Merge "Drop RpcProxy usage from MetadataPluginApi"