This adds the functional notification sample test for the
aggregate.cache_images.start and aggregate.cache_images.end
versioned notifications.
I also added a comment to the docs builder code since it took
me a bit to figure out how to get the notification sample
linked into the docs, and for whatever reason figured that out
by looking through code rather than our nicely detailed docs
that already explain it.
Part of blueprint image-precache-support
Change-Id: I0869979a1b8a0966f0e7b49e5a5984f76d7d67cd
d50efc337c