Python 3: fix a syntax error
Python 3: fix a syntax error

"raise AttributeError, name" is invalid in Python 3.

Change-Id: Id61bd3747f49c2bd810cbfeae56506e7ed9d2bd0

metacloud/python-openstackclient,varunarya10/python-openstackclient,redhat-openstack/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,varunarya10/python-openstackclient,redhat-openstack/python-openstackclient,dtroyer/python-openstackclient,BjoernT/python-openstackclient,derekchiang/python-openstackclient,BjoernT/python-openstackclient,derekchiang/python-openstackclient,metacloud/python-openstackclient,openstack/python-openstackclient