zuul-merger: increase maximum number of opened files

This change tries to mitigate the "Too many open files" error happening
on busy deployment. This is using the same limit as in puppet-zuul
module.

Change-Id: I277865b06b98665e7c577703faf9369b90eec183

files/etc/init.d/zuul-merger

@@ -50,6 +50,7 @@ do_start()
 
     mkdir -p /var/run/$NAME
     chown $USER /var/run/$NAME
+    ulimit -n 8192
     if [ -f $PIDFILE ]; then
         return 3
     fi

files/etc/systemd/system/zuul-merger.service

@@ -6,6 +6,7 @@ After=syslog.target network.target
 Type=simple
 User=zuul
 Group=zuul
+LimitNOFILE=8192
 EnvironmentFile=-/etc/sysconfig/zuul-merger
 ExecStart=/bin/sh -c "${PREFIX}/bin/zuul-merger -d"