Add the possibility to pass the zuul url

This patch adds the zuul_url option in zuul conf file
in order to pass ZUUL_URL to Jenkins, which will be used
by devstack-vm-gate-wrap.sh.
Documentation added in launchers.rst and zuul.rst
explaining how this new option works.

Change-Id: I840423cc06fdfdacd301d30be3e0b3e589e563e9
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 6b5d7c9..afe4cf6 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -139,6 +139,10 @@
   is included).  Defaults to ``false``.
   ``job_name_in_report=true``
 
+**zuul_url**
+  URL of Zuul's git repos, accessible to test workers.  
+  Usually "http://zuul.example.com/p".
+
 smtp
 """"