Fix readthedocs document build job for Deckhand
Fix readthedocs document build job for Deckhand

Currently readthedocs build is failing for DH because it runs
pip install -rtest-requirements.txt [0] when test-requirements.txt
doesn't include oslo.policy. And readthedocs can't run tox -e docs
which is what is intended to be run for documentation generation.

[0] https://readthedocs.org/projects/deckhand/builds/6351568/

Change-Id: I4ad93e41b0410c547c9fec7d6d38f3b7e7f6829f

att-comdev/deckhand,att-comdev/deckhand