This adds the possibility to retrieve prometheus or aetos endpoint
information from keystone. Aetos is expected to have the endpoint
registered (Aetos's devstack plugin already does this) and admins
can register plain prometheus in Keystone with a simple command.
Previous functionality with using /etc/openstack/prometheus.yaml or
env variables is kept unchanged.
Change-Id: I20eb5858244f1202ab8bc1fa26bb46b41d927ac0
e81c54e497