Fix misleading message about statsd not installed

If the stats object is none we actually didn't configure it. In case
statsd is configured but not installed the scheduler will crash
earlier with a stack trace.

Change-Id: Id9976d78ca41ed4d2ffb164942048273cff7e07f
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index b978979..c3f2f23 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -823,8 +823,7 @@
         if self.statsd:
             self.log.debug("Statsd enabled")
         else:
-            self.log.debug("Statsd disabled because python statsd "
-                           "package not found")
+            self.log.debug("Statsd not configured")
         while True:
             self.log.debug("Run handler sleeping")
             self.wake_event.wait()