Add a graphite key for all jobs in a pipeline

We have a graph on our status page showing all of the jobs Zuul
launched, but it's built from more than 1000 graphite keys which
is a little inefficient.  Add a key for convenience that rolls
up all of the job completions in a pipeline, so that such a graph
can be built with only about 10 keys.

Change-Id: Ie6dbcca68c8a118653effe90952c7921a9de9ad1
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index b2d3be9..45e55fb 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -339,6 +339,8 @@
                     dt = int((build.end_time - build.start_time) * 1000)
                     statsd.timing(key, dt)
                 statsd.incr(key)
+                key = 'zuul.pipeline.%s.all_jobs' % build.pipeline.name
+                statsd.incr(key)
         except:
             self.log.exception("Exception reporting runtime stats")
         self.result_event_queue.put(('completed', build))