The deprecation of username in favor of user-name has been reverted in
python-keystoneclient in [1], we should keep this in sync here.
This is a cherry-pick of [2], the patch from [1] has been backported to
stable/liberty in [3], so we should aim to be consistent here.
[1] https://review.openstack.org/247574
[2] https://review.openstack.org/287754
[3] https://review.openstack.org/269364
Change-Id: Ia82536ecebfc5874add36f6fbc349d9a9cbf3a08