Previous to the connection work zuul wouldn't stop triggers or therefore
the gerrit watcher on reload.
Reloading the gerrit connection causes challenges for the cache
management which are currently not handled and likely affecting memory
usage.
Rather than support dynamically reloading all of zuul.conf, just reload
the layout and logging and require a full restart for connection
changes to take effect.
This isn't a feature regression as particular changes to zuul.conf
weren't previously picked up (for example, gerrit credentials).
Change-Id: Iff77fbca6ab15c9636f6ae8f12bc1bad78f79aa9
c4997159bb