Reintroduces fake_rabbit config option

This change reintroduces the fake_rabbit only for backward compatibility,
but mark it as deprecated.

Now, to use the kombu in-memory driver (that is not thread safe) we must
set the transport_url config option to 'kombu+memory:////" or the
rpc_backend to kombu+memory.

Or we can use the fake driver of oslo.messaging by setting the
transport_url to 'fake:///' or the rpc_backend to 'fake'

This is effectively reverting commit bcb3b23b8f6e7d01e38fdc031982558711bb7586.

Closes-bug: #1399085

Change-Id: I7b6fb3811fc6f695f75ecd350e04e69afd26c428