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
file.
Change-Id: I35fb782b7bad4c046930926c43b0f7b3c1bb07b7
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
jobs.
Align it to the form used in other repos.
Change-Id: I49bb60a05cf63247091ee72591a799f6efc7c84b