Add a Sphinx Directive to automatically list and document drivers

[Vincent Françoise]

As we have made Watcher pluggable, we need to be able to automatically document each entry point dynamically instead of having to do it manually.

So the idea would be to have a Sphinx directive which would import the docstring of each available entry point.

[Vincent Françoise (vincent-francoise)]

Quick draft available at https://gist.github.com/v-francoise/08bbe0cb375b8c4a55fa

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/279084

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/279084
Committed: https://git.openstack.org/cgit/openstack/watcher/commit/?id=98a65efb163f8c00f4af06a45f7dfa51633b31cc
Submitter: Jenkins
Branch: master

commit 98a65efb163f8c00f4af06a45f7dfa51633b31cc
Author: Vincent Françoise <email address hidden>
Date: Thu Feb 11 14:49:19 2016 +0100

    RST directive to discover and generate drivers doc

    This patchset introduces a new custom directive called 'drivers-doc'
    which loads all available drivers under a given namespace and import
    their respective docstring into the .rst document.

    This patchset also contains some modification/addition to the
    docstring of these drivers to make the final document complete.

    Change-Id: Ib3df59fa45cea9d11d20fb73a5f0f1d564135bca
    Closes-Bug: #1536218
    Closes-Bug: #1536735