Drop RpcProxy usage from mlnx plugin

Drop RpcProxy usage from mlnx plugin

This patch drops usage of the RpcProxy compatibility class from the
mlnx plugin.  The equivalent oslo.messaging APIs are now used instead.

Note that one test case was dropped, but only because it did not
provide any actual additional test coverage.

Also note that the base rpc version is set to '1.0' instead of '1.1'
as it was in the original code.  Setting the base to X.0 is the right
pattern.  Methods that require a newer version should specify it in
their implementation.  According to the API history comment here, the
methods in this class were not the ones changed in '1.1', so they
don't specify '1.1' as a requirement.

Part of blueprint drop-rpc-compat.

Change-Id: Ia8174b3e3415aa1e8588d908117e79a604e63881