Add python-six to requirements
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

cernops/keystone,dims/keystone,JioCloud/keystone,rodrigods/keystone,openstack/keystone,promptworks/keystone,jamielennox/keystone,roopali8/keystone,openstack/keystone,rushiagr/keystone,rajalokan/keystone,jonnary/keystone,mahak/keystone,himanshu-setia/keystone,dstanek/keystone,derekchiang/keystone,openstack/keystone,klmitch/keystone,reeshupatel/demo,blueboxgroup/keystone,rushiagr/keystone,promptworks/keystone,reeshupatel/demo,takeshineshiro/keystone,cloudbau/keystone,cloudbau/keystone,ilay09/keystone,reeshupatel/demo,maestro-hybrid-cloud/keystone,ntt-sic/keystone,JioCloud/keystone,vivekdhayaal/keystone,idjaw/keystone,cloudbau/keystone,idjaw/keystone,UTSA-ICS/keystone-kerberos,takeshineshiro/keystone,rajalokan/keystone,nuxeh/keystone,jumpstarter-io/keystone,dims/keystone,maestro-hybrid-cloud/keystone,blueboxgroup/keystone,nuxeh/keystone,ntt-sic/keystone,mahak/keystone,ging/keystone,ilay09/keystone,jumpstarter-io/keystone,derekchiang/keystone,promptworks/keystone,dsiddharth/access-keys,dstanek/keystone,mahak/keystone,derekchiang/keystone,roopali8/keystone,jumpstarter-io/keystone,dstanek/keystone,UTSA-ICS/keystone-kerberos,vivekdhayaal/keystone,MaheshIBM/keystone,dsiddharth/access-keys,vivekdhayaal/keystone,ajayaa/keystone,ntt-sic/keystone,rushiagr/keystone,dsiddharth/access-keys,klmitch/keystone,nuxeh/keystone,ilay09/keystone,ajayaa/keystone,rajalokan/keystone,jamielennox/keystone,rodrigods/keystone,MaheshIBM/keystone,himanshu-setia/keystone,jonnary/keystone,ging/keystone,cernops/keystone