Add zuul.timeout variable to jobs

The existing devstack-gate system uses this.

Change-Id: I7e324397dfe22ec11c157dda13d7f59de40211f7
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index 4c8b784..837fb17 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -248,6 +248,10 @@
 
       The name of the current Zuul tenant.
 
+   .. var:: timeout
+
+      The job timeout, in seconds.
+
    .. var:: jobtags
 
       A list of tags associated with the job.  Not to be confused with
diff --git a/zuul/executor/client.py b/zuul/executor/client.py
index fcb6bb2..be41186 100644
--- a/zuul/executor/client.py
+++ b/zuul/executor/client.py
@@ -167,6 +167,7 @@
                            voting=job.voting,
                            project=project,
                            tenant=tenant.name,
+                           timeout=job.timeout,
                            jobtags=sorted(job.tags))
         if hasattr(item.change, 'branch'):
             zuul_params['branch'] = item.change.branch