update cliff requirement to >=1.7.0
update cliff requirement to >=1.7.0

requirements should keep up with global requirements, currently we
spin cliff to 1.6.1, which will break grenade test for openstack
projects.

Change-Id: If9659d63a736238b504eaa0d6ac8a662ffd1e0fc
Closes-Bug: #1388414

cloudkeep/python-barbicanclient,akash1808/python-barbicanclient,cloudkeep/python-barbicanclient,akash1808/python-barbicanclient,openstack/python-barbicanclient