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
8 files changed