Replace dict.iteritems() with dict.items()
Replace dict.iteritems() with dict.items()

Following Python 3 guide https://wiki.openstack.org/wiki/Python3
Thus, we should replace task.iteritems() with task.items()

Change-Id: If617c3a87836930dc78a4ce7dd45a9b7804e0d24

openstack/python-glanceclient,openstack/python-glanceclient