Adapting local docs build and remove build warnings

Adapting local docs build and remove build warnings

The OPNFV docs project will remove its submodules and enable local docs
builds. This patch prepares the Dovetail repo according to the official
transition guide:

https://docs.opnfv.org/en/latest/how-to-use-docs/local-build-transition.html

This patch also applies syntactical changes which eliminate the sphinx
doc build warnings.

Change-Id: Ief8fd2d1c3e39b232d214a9ab392879ee4a492c8
Signed-off-by: Georg Kunz <georg.kunz@ericsson.com>