Update six dependency
Update six dependency

The version of six specified in nova's requirements list
conflicts with the version used by the python-keystoneclient
requirement. This change updates the settings for six so
they match.

Change-Id: I3082090f5d3849a7fbebf515c3d743cd8c3e9c43

TieWei/nova,rickerc/nova_audit,TieWei/nova,SUSE-Cloud/nova,rickerc/nova_audit,SUSE-Cloud/nova