Added the beta hostname to local_domains to make exim use mw_verp_api

The bounce needs to get HTTP POSTed to the webserver ( currently running
on mediawiki-verp in labs ) which will not happen until mediaiwki-verp.
wmflabs.org is there in the local_domains list. This patch adds the hostname
to the local_domains, and make sure only labs is affected

Change-Id: I0600d323c80743ce01d284740efd4fc6217739e5
Merge "Added the beta hostname to local_domains to make exim use mw_verp_api" into production