Add openstackclient dependency back
Add openstackclient dependency back

Change I9543988830eeeac0d09facebabdeed9a968ba3f0 used osc-lib
to replace OpenStackClient(OSC), and removed OSC from
requirement.txt. This will cause this plugin client unable to
work until users install OSC themselves.

osc-lib aims to provide common utils that all OSC plugins may
use. At some point, some uses will not want OSC automagically
pulled in, and osc-lib allows that. But for those who want to
install OSC automatically, we should add the dependency back.

Change-Id: I390fd17ff6b856990ee0de8bfb0954e2e85a33bb

openstack/python-saharaclient