Drop RpcProxy usage from MetadataPluginApi
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

gkotton/vmware-nsx,gkotton/vmware-nsx