Make isJobRegistered for gearman optional

With openstack-infra we actually validate our jobs exist before
merging them into our zuul configuration. As a result, the
registration check in zuul is redundant.  So provide a way for the
check to be enable / disabled.

Change-Id: I06f2221770e04f958ce7ac4cfe8d5d92d7164cac
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 98e4bb8..07b777a 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -49,6 +49,11 @@
   Port on which the Gearman server is listening.
   ``port=4730``
 
+**check_job_registration**
+  Check to see if job is registered with Gearman or not. When True
+  a build result of NOT_REGISTERED will be return if job is not found.
+  ``check_job_registration=True``
+
 gearman_server
 """"""""""""""