Allow executor to core dump

ansible-playbook is getting hit with sigsegv; allow the process
to core dump so we might obtain clues.

Change-Id: Ice0bbd4abea2997ee56a92544128fd136dcef048

files/zuul-executor.init

@@ -47,6 +47,7 @@ do_start()
     mkdir -p /var/run/$NAME
     chown $USER /var/run/$NAME
     ulimit -n 8192
+    ulimit -c unlimited
     if [ -f $PIDFILE ]; then
         return 3
     fi