[Reviewer: Ellie] Configure each ChronosConnection with a callback_host, so that each subs...

...equent call only needs a path

This addresses part of your review feedbac, in that it's useful for not hardcoding "localhost:9888" everywhere.

Worth looking at https://github.com/Metaswitch/sprout/commit/72bec69dc1e95bf5144df990c9aabd7c67c32c25 alongside this.