Fix Devstack and Magnum-ui interaction
Fix Devstack and Magnum-ui interaction

Devstack always installs test-requirements, but we do not expect it for
Magnum UI project. Moreover, such installation started failing with
PIP versions 8.x
Change that triggered bug is Devstack change at [1].

[1] I49658ce4056c773943321270defd461bbf3e9fb9

Change-Id: I02d24d9c11988d78a59f198d2048e6861f78d83f
Closes-Bug: #1540188

openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui