We currently do not support overriding the class being
instantiated in the RPC helper functions, this adds that
support so that projects that define their own classes
that inherit from oslo.messaging can use the helpers.
For example neutron utilizes code from neutron-lib that
has it's own RPCClient implementation that inherits from
oslo.messaging, in order for them to use for example
the get_rpc_client helper they need support to override
the class being returned. The alternative would be to
modify the internal _manual_load variable which seems
counter-productive to extending the API provided to
consumers.
Change-Id: Ie22f2ee47a4ca3f28a71272ee1ffdb88aaeb7758
687dea2e65