Code review comment for lp://staging/~boyang-niu/maas/enable_custom_images

To enumerate on what the smaller steps are:
 * Consuming "simplestreams" data, as juju does, to get images for download
 * Removing the hard-coded enum based on that work
 * Consuming from a custom simplestreams feed, for people who want their
own kernel images
 * Making cluster controllers synchronise this data rather than the
region (this one needs more discussion)
 * and of course, the UI changes