Rather than implement functional OpenStack testing as a devstack
plugin, use devstack as a black-box OpenStack. This allows us
to move nodepool to a new Zuul tenant without depending on the
devstack job and all its associated projects. It will also let
us replace devstack with something potentially lighter weight in
the future.
The job permits customized settings for what images to build, so
that the DIB project can inherit from it and make child jobs for
each of the operating systems it cares about.
Change-Id: Ie6bc891cebd32b3d1bb646109f13ac2fd383bba5
007f7e0b08