Instead of using tox-install.sh, we can install
horizon now directly from pypi - or use the current branch in CI system
using tox-siblings.
Create doc/requirements.txt for doc requirements as well, it's use
currently only for releasenotes. Remove obsolete oslosphinx requirement.
Remove docs environment - this project has no docs, only releasenotes.
Update tox.ini for this change and add horizon to requirements
file.
Change-Id: I72a942149a88183a32afc0440ead7b927bdc93b5
ca02545ad6