Allow and document use of the uri module from localhost

The rtfd hook job just does an empty POST to a URI. There's no need to
allocate a node for that, we can just make REST calls from the executor.

Also, there is enough going on here that it needs to be documented. Add
a documentation section to the developer docs about what we're doing
with our ansible plugins. In support of that, add a simple sphinx domain
for ansible to allow us to easily link to upstream ansible documentation for
modules.

Change-Id: I9b0be1018388db7361aec10f30a70437de555615
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 7c0d587..38c1689 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -29,6 +29,7 @@
     'sphinx.ext.autodoc',
     'sphinxcontrib.blockdiag',
     'sphinxcontrib.programoutput',
+    'zuul.sphinx.ansible',
     'zuul.sphinx.zuul',
 ]
 #extensions = ['sphinx.ext.intersphinx']