Add missing oslo-config to requirements

openstackclient uses these oslo-config but it is not in
list of requirements. That results in a stacktrace while
using the cli.

Change-Id: I9b3c8e491ce09624524cf8b3ddcafcf5953d9ab6

Author: Abhishek Chanda

requirements.txt

@@ -7,6 +7,7 @@ six>=1.7.0
 Babel>=1.3
 cliff>=1.7.0  # Apache-2.0
 cliff-tablib>=1.0
+oslo.config>=1.6.0  # Apache-2.0
 oslo.i18n>=1.0.0  # Apache-2.0
 oslo.utils>=1.2.0                       # Apache-2.0
 oslo.serialization>=1.2.0               # Apache-2.0