Avoid oslo.messaging 1.5.0

1.5.0 dropped the fake_rabbit option which was still used by some
projects.  It was reintroduced in 1.5.1.

Change-Id: I47138da02c58073b03e3fb40537cc6f0b6a94a3c
Related-bug: #1399085
Merge "Avoid oslo.messaging 1.5.0"