Add job tags

This allows us to add arbitrary string 'tags' to jobs.  These may
then be inspected inside of parameter functions for any purpose.
In OpenStack, we will likely pass these through untouched to the
the build so that they can be picked up by the logstash worker
and we can record extra metadata about jobs.

Change-Id: Ibc00c6d30cdfe4678864adb13421a4d9f71f5128
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index b5b8d7b..d8d72e6 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -765,6 +765,12 @@
   Boolean value (``true`` or ``false``) that indicates whatever
   a job is voting or not.  Default: ``true``.
 
+**tags (optional)**
+  A list of arbitrary strings which will be associated with the job.
+  Can be used by the parameter-function to alter behavior based on
+  their presence on a job.  If the job name is a regular expression,
+  tags will accumulate on jobs that match.
+
 **parameter-function (optional)**
   Specifies a function that should be applied to the parameters before
   the job is launched.  The function should be defined in a python file