config-changed hook works properly with async updateConfig

This was originally meant as a fix for the port config option, but that's due to elevated perms.  That is another issue that will be fixed by exposing http interface.  Instead, this assures that config-changed operations take place in the right order, thanks to async updateConfig.

To QA:

* deploy
* `juju set ghost port=5523`
* Ensure ghost is visible on port 5523