config: use hostname as default value for librato source
config: use hostname as default value for librato source

This makes it easier to set up Librato in places where it's hard to set per-host
settings (such as with our current Chef recipe).

henrikhodne-abandoned/worker,henrikhodne-abandoned/worker,solarce/worker,henrikhodne-abandoned/worker,solarce/worker,Acidburn0zzz/worker,Acidburn0zzz/worker,solarce/worker,travis-ci/worker,travis-ci/worker,travis-ci/worker,Acidburn0zzz/worker