[TAV-508] Allow workers to run against different servers.

This allows us to add in a more taverna servers for scaling.
[TAV-511] Don't save run log if it's an empty file.

This confuses Paperclip's validators.
Merge branch 'next'

* next:
  [TAV-511] Don't save run log if it's an empty file.
  [TAV-508] Allow workers to run against different servers.