Stop installing pip packages on zuul.

Let setup.py install handle it instead.

Change-Id: I7f143dcc4c19af267ef820973184c45e2020a3de
Reviewed-on: https://review.openstack.org/25551
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins