Add keystoneclient to requirements.txt
Add keystoneclient to requirements.txt

At some point on 2018-02-02, all tripleo-common tox python jobs started
failing with:

2018-02-02 15:05:50.952672 | ubuntu-xenial | ImportError: No module
named keystoneclient

We did't have python-keystoneclient in requirements.txt, though we
probably should. We must have been implicitly getting it from somewhere
else previously and something must have changed to where it's now
explicitly required.

Change-Id: I272292f2f411d1195e5986f0f6a24b379fbf2b6e
Closes-Bug: #1747160

openstack/tripleo-common,openstack/tripleo-common