This script is unnecessarily complex for what it is doing, which is
essentially pre-seeding the source-repositories cache for devstack.
As we can see from I41e81d6bac98875eecde2376e0865784626e11a8 it's very
confusing having large parts of the source-repositories script
copy-pasted as a separate element and has led several of us down the
wrong path.
Strip this script back to the simple thing it is doing, which is
checking out/updating the source-repositories devstack cache.
I have tested this by building an image with a warm cache and with a
cold cache, in both cases the checkout was found and the list of
images to cache in 55-cache-devstack-repos was found.
Change-Id: I6c686312de102cbe438585e26bf6986e06b6f41c
abbbf317ce