Revert "Encapsulate getting image for a deployment."

This reverts commit 2f2cb8a3993f9b18290b2644b031348559971c2c.

This feature removed the ability to use locally cached images
from the devtest_* scripts. Also adding top level --download-images
flags to all of the devtest scripts globally isn't something we
want to do. Ideally users would download images of their choosing
ahead of time and just simply set USE_CACHE=1. Much simpler and
can support a variety of download/caching mechanisms.

Change-Id: If788c2772f41fa4c41ea81a94e7c4335c1566158