This adds a robots.txt that kindly asks bots to not crawl anything on
zuul. We've seen soem bots crawling which leads to them trolling the
build logs which seems like overkill and increases bandwidth usage in
our donor clouds. Ask them to stop and quiet everything down a bit.
Change-Id: I88d85c7a51159b5b020aa179e24acec55fb42931
7ad3125814