Add hpcloud-az3 devstack-gate provider.

Add hpcloud az3 as another devstack-gate Jenkins' slave provider. This
will further diversify the devstack-gate availability zones decreasing
the effects of AZ specific problems.

Change-Id: Ie35574ff6d06753d69b596c59234b26437ad5f7e
Reviewed-on: https://review.openstack.org/21902
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins