Additionally install test-requirements with pip_install
Additionally install test-requirements with pip_install

When moving to Python 3, we also need to install test-requriements
to allow projects to install any python 3 test dependencies they
might be missing otherwise.

Change-Id: I2d19aa2f7ec8de869a82aa7764ab72cc8693101f

dtroyer/devstack,thomasem/devstack,Mahito/devstack,samgoon/devstack,noironetworks/devstack,prithivm/branch01,openstack-dev/devstack,pombredanne/devstack,varunarya10/devstack,prithivm/branch01,bigswitch/devstack,dirkmueller/devstack,bswartz/devstack,openstack-dev/devstack,bswartz/devstack,group-policy/devstack,thomasem/devstack,noironetworks/devstack,mssumanth/devstack,bigswitch/devstack,group-policy/devstack,olivierlemasle/devstack,ozamiatin/devstack,ozamiatin/devstack,nawawi/openstack,Mahito/devstack,samgoon/devstack,varunarya10/devstack,richliu/devstack,avvocatodemarchis/devstack,dirkmueller/devstack,pombredanne/devstack,nawawi/openstack,avvocatodemarchis/devstack,olivierlemasle/devstack,mssumanth/devstack,richliu/devstack,dtroyer/devstack