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/requriements.txt for doc requirements as well.
Update tox.ini for this change and add horizon to requirements
The tox_install.sh here behaves a bit differently than the others, which
is causing some problems when trying to rework some of the shared gate
Align it to the form used in other repos.