Adds fixture package from oslo

The mock library is added to test-requirements.txt since the
mockpatch fixture requires it.

Change-Id: I1b5b0c75f256382a685fceb2117db6d5b18d8c4f
(cherry picked from commit 07aa0a91a120c1b07cbfe800ec0f78abb81a7271)
Merge "Adds fixture package from oslo" into stable/havana