Build hpcloud devstack precise and trusty nodes off of dib built images.
Last time we attempted similar we ran into bugs with nodepool not
handling labels with both snapshot and dib images. These issues should
be corrected now so that rax and hpcloud can have two different images
sources as configured by this change.
Change-Id: Ia6d67de14fbd681e946c0178f77c0a8e74c7dabc