OpenDev infra only keep around the latest two Fedora releases in their
mirrors. Probe for the image from the local test mirror, but if not
found, fallback to upstream. This will be much less reliable, but can
avoid gate breakage until new images can be used.
Also, use endpoint_type when creating keystoneclient
Keystone admin endpoint has been removed from devstack with[1].
This would use the public endpoint by default.
Change-Id: I96ab14871ee8c5d5b83cc0cd4abc840ef0218ca8