Default scheduler pid file path to zuul-scheduler

Now that we are calling this daemon 'zuul-scheduler' we should default
its pid path to /var/run/zuul-scheduler/zuul-scheduler.pid.

Change-Id: I4dd263dc34def5b41a926cf5248f7cc35b3c5d9f
diff --git a/zuul/cmd/scheduler.py b/zuul/cmd/scheduler.py
index e5497dc..7d225a1 100755
--- a/zuul/cmd/scheduler.py
+++ b/zuul/cmd/scheduler.py
@@ -235,7 +235,7 @@
     if scheduler.config.has_option('zuul', 'pidfile'):
         pid_fn = os.path.expanduser(scheduler.config.get('zuul', 'pidfile'))
     else:
-        pid_fn = '/var/run/zuul/zuul.pid'
+        pid_fn = '/var/run/zuul-scheduler/zuul-scheduler.pid'
     pid = pid_file_module.TimeoutPIDLockFile(pid_fn, 10)
 
     if scheduler.args.nodaemon: