Scheduler: Reduce log level for non-configured projects

If Gerrit is hosting also projects that are not using this
Zuul instance it is annoying to get warnings about
non-configured projects for each received Gerrit event.
Therefore the log level is being reduced to debug.

Change-Id: I535a4f4778c1413b6d30f81980769128bbaba6db
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index 9effcb8..d06693e 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -810,7 +810,7 @@
         try:
             project = self.layout.projects.get(event.project_name)
             if not project:
-                self.log.warning("Project %s not found" % event.project_name)
+                self.log.debug("Project %s not found" % event.project_name)
                 return
 
             for pipeline in self.layout.pipelines.values():