Local Documentation Builds

Local Documentation Builds

This adds configuration for performing local documentation builds
with the following simple command:

tox -e docs

This patch is part of our documentation effort to Move away from
submodules
https://docs.opnfv.org/en/latest/how-to-use-docs/local-build-transition.html

Once merged docs will be viewable at opnfv-nfvbench.readthedocs.io

Change-Id: I7c877847ec23c4cfc44ff7f106ed2acc65688a41
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>