Add trigger-readthedocs job

It's really easy to trigger a build on readthedocs. It doesn't even need
a node, since it's just a single POST.

Change-Id: I8c576c7a26ab123bd70f7a025d8a49a5cb3e2432

playbooks/python/readthedocs.yaml

@@ -0,0 +1,6 @@
+- hosts: localhost
+  tasks:
+    - name: Trigger readthedocs build URI
+      uri:
+        method: POST
+        url: 'https://readthedocs.org/build/{{ zuul.project.short_name }}'

zuul.yaml

@@ -134,6 +134,13 @@
       at least a ``username`` and ``password`` attribute.
     post-run: playbooks/python/upload-pypi
 
+- job:
+    name: trigger-readthedocs
+    description:
+      Send a trigger to the readthedocs url to tell it to build docs
+    nodes: []
+    run: playbooks/python/readthedocs
+
 - job:
     name: multinode
     description: |