Add logging handler for openstack_auth in the tests

Without it, log output below ERROR level can also be displayed in the
tests output.

Change-Id: I342b20fc26468a41843496f11db3e0079b50505c
Closes-Bug: #1218462
(cherry picked from commit 8665768523f0afa057a485a7b0194c4b71e3a627)
Merge "Add logging handler for openstack_auth in the tests" into stable/icehouse