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

Author: Colleen Murphy

requirements.txt

@@ -6,6 +6,7 @@ six>=1.7.0
 
 Babel>=1.3
 cliff>=1.7.0  # Apache-2.0
+cliff-tablib>=1.0
 oslo.i18n>=1.0.0  # Apache-2.0
 oslo.utils>=1.0.0                       # Apache-2.0
 oslo.serialization>=1.0.0               # Apache-2.0