Make zuul-merger default pidfile match init script

The init script for the zuul-merger file does not look for the
zuul-merger PIDFILE in the same default location that the zuul-merger
uses. Make them both look at /var/run/zuul-merger/zuul-merger.pid.

Change-Id: Ia5829ef3887a5651d23a80dbd00e76972c7cf6a7
Merge "Make zuul-merger default pidfile match init script"