untangle fetch_sources and setup_sources roles

[evgeni]

"We have two roles right? fetch_sources and setup_sources ? I think that is breeding a lot of my confusion. I recall what the fetch_sources role was born out of a bit more now and I think maybe we need to re-consider it's name? or maybe it's purpose. That is, for something that makes you think it fetches sources, it does updating at the same time. Not your design, but I wondered if you had thoughts from doing this split out of how it could be renamed or broken out such that it makes the responsibility of the role cleaner."

Originally posted by @ehelms in #216 (comment)

Probably they need splitting and renaming to better expose what they do.

[ekohl]

As an aside: I think the update role should also only include roles. Currently it includes tasks to do git annex + commit as well.