Add change_url to zuul dict passed into inventory

In order to emit a message in a job like "this job was started by
https://review.openstack.org/#/c/501485/" we need to have access to
that information.

Change-Id: I05d1e9a944be914375b9ed2affa0f1e73c8e773d
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index 4c8b784..160a9e0 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -299,6 +299,12 @@
 
          The identifier for the change.
 
+      .. var:: change_url
+
+         The URL to the source location of the given change.
+         E.g., `https://review.example.org/#/c/123456/` or
+         `https://github.com/example/example/pull/1234`.
+
       .. var:: patchset
 
          The patchset identifier for the change.  If a change is
@@ -328,6 +334,12 @@
       The patchset identifier for the change.  If a change is revised,
       this will have a different value.
 
+   .. var:: change_url
+
+      The URL to the source location of the given change.
+      E.g., `https://review.example.org/#/c/123456/` or
+      `https://github.com/example/example/pull/1234`.
+
 Branch Items
 ++++++++++++