Add noop job to config validator

With the addition of the internal "noop" job, the config validator
should know that it exists when checking that a config file
references valid jobs.

Change-Id: Icea4def074fd9a1152966f2c768e30d98d432dfd

zuul/cmd/server.py

@@ -137,6 +137,7 @@ class Server(object):
         if not os.path.exists(path):
             raise Exception("Unable to find job list: %s" % path)
         jobs = set()
+        jobs.add('noop')
         for line in open(path):
             v = line.strip()
             if v: