Fix RPC responses to allow None response correctly.
Fix RPC responses to allow None response correctly.

Fixes bug 897155

Also adds a new fake rpc implementation that tests use by default.
This speeds up the test run by ~10% on my system.  We can decide to
ditch fake_rabbit at some point later..

Change-Id: I8877fad3d41ae055c15b1adff99e535c34e9ce92

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects