Add a dedicated slave to launch devstack nodes.

So that we can take this load off of the jenkins master.

Change-Id: Id66efede75fab9977013e69f3415a53dc239b0ce
Reviewed-on: https://review.openstack.org/22379
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins