Add cliff-tablib to requirements
Add cliff-tablib to requirements

cliff-tablib gives cliff the ability to format list and show output in
html, json, or yaml (http://cliff-tablib.readthedocs.org/). This patch
adds cliff-tablib to requirements.txt so that it can be installed along
with cliff.

Change-Id: I4daab97642482e6f40cd8209ff5edd9c680092c0

openstack/python-openstackclient,dtroyer/python-openstackclient,redhat-openstack/python-openstackclient,openstack/python-openstackclient,varunarya10/python-openstackclient,BjoernT/python-openstackclient,BjoernT/python-openstackclient,dtroyer/python-openstackclient,redhat-openstack/python-openstackclient,varunarya10/python-openstackclient