Match other python-*client prettytable dependency
Match other python-*client prettytable dependency

The other openstack clients use prettytable>=0.6,<0.7. For several
reasons (primarily a lack of support in pip), we will match that here.

Change-Id: Id4fb08ae48a65666014c96a22baefe46a771b002

metacloud/python-neutronclient,metacloud/python-neutronclient