Remove httplib2 dependency

Once upon a time, the python-*client libraries were primarily built to
use httplib2. They have subsequently shift to using requests and thus
urllib3. The horizon test helpers code was maintaining a reference to
httplib2 as it intercepted errant library calls that were not mocked.

httplib2 is not actively maintained and OpenStack is moving to remove it
as a dependency. See http://lists.openstack.org/pipermail/openstack-dev/2016-March/089225.html
for more details.