Allow RPC_API_NAMESPACE on RpcProxy objects

This allows one to subclass RpcProxy for use with a particular
namespace without having to repeatedly pass the namespace into the
make_namespaced_msg() method.  This new RPC_API_NAMESPACE attribute will
be used for the make_msg() call.  It is defaulted to None to match previous
behavior.

Change-Id: Ied62df839cda0be6f9f9b060bbfc22e086f61be8