Adds github triggering from status updates

This adds support for triggering on github status updates.

Config schema for the github trigger has been updated to accept a list
of statuses, in the "github_user:context:status" format.

Change-Id: I15aef35716ddbcd1e66f84a73d27ca2689c936e4
Co-Authored-By: Jesse Keating <omgjlk@us.ibm.com>
Signed-off-by: Adam Gandelman <adamg@ubuntu.com>
diff --git a/doc/source/triggers.rst b/doc/source/triggers.rst
index f73ad2f..41a56a0 100644
--- a/doc/source/triggers.rst
+++ b/doc/source/triggers.rst
@@ -133,6 +133,8 @@
 
     *push* - head reference updated (pushed to branch)
 
+    *status* - status set on commit
+
   A ``pull_request_review`` event will
   have associated action(s) to trigger from. The supported actions are:
 
@@ -165,6 +167,12 @@
   strings each of which is matched to the review state, which can be one of
   ``approved``, ``comment``, or ``request_changes``.
 
+  **status**
+  This is only used for ``status`` actions. It accepts a list of strings each of
+  which matches the user setting the status, the status context, and the status
+  itself in the format of ``user:context:status``.  For example,
+  ``zuul_github_ci_bot:check_pipeline:success``.
+
   **ref**
   This is only used for ``push`` events. This field is treated as a regular
   expression and multiple refs may be listed. Github always sends full ref