Make base_image selection more intelligent

1. #base_image should return an image object
2. if an appropriate base_image is not found, announce the search failure
and fall back to the default image selection (i.e., without regard to the
"name" parameter)
Merge branch 'master' into ha-feature-trusty
Append cookbooks SHA to the end of the image name

This way, travis-worker can pick up our templates with SHA
without changing its logic.

Before: travis-update-standard-efe0364-2014-07-30-10-30
After:  travis-update-standard-2014-07-30-10-30-efe0364

See https://github.com/travis-ci/travis-worker/blob/9033ee176e6680046e2360b68270183fcfa2b04d/lib/travis/worker/virtual_machine/blue_box.rb#L25
Merge branch 'master' into ha-feature-trusty