Instead of using tox-install.sh, we can install horizon and neutron
now directly from pypi - or use the current branch in CI system using
tox-siblings.
Update tox.ini for this change and add horizon to requirements file.
Change-Id: I898fb222f5b36714c61e60c222e8b57ddea99df6
pip_install.sh is renamed to tox_install.sh to align with other projects.
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: Ie35d2b33f4bfaa8383c7af9e5ca489b61f7da073