Update solum devstack job to install packages from test-requirements.txt

Somehow in devstack job all the packages listed in test-requirements are
not installed. There is an explanation [1] for this in the dev list.

So as suggested by Clark Boylan in the thread, just install them in
pre_test_hook.

[1] http://www.mail-archive.com/openstack-dev@lists.openstack.org/msg12531.html

Change-Id: I39cdfb4544fd81db79c02909962f9d4bf1ed879f
Merge "Update solum devstack job to install packages from test-requirements.txt"