Update python-saharaclient to 0.7.6 version

The updated version contains the following
new functionality:
* keystone v3 CLI
* support for filtering list() calls
* unit tests for python bindings

Change-Id: I3bec5236d9db846aa3ffb311015018729d3735a0
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 "Update python-saharaclient to 0.7.6 version"
Merge "Avoid oslo.messaging 1.5.0"