Add python-six to requirements

missing python-six minimum version from the requirements.txt
set the min version to 1.4.1, without six recent versions of
keystone fail to start with a critical 'module' object has no
attribute 'add_metaclass' error

Change-Id: Ibfe2c9ec2457d3131dc9b6760e343c36ab8a3c38
Closes-Bug: #1237089