Avoid tox_install.sh for constraints support

We do not need tox_install.sh, pip can handle constraints itself
and install the project correctly. Thus update tox.ini and remove
the now obsolete tools/tox_install.sh file.

This follows https://review.openstack.org/#/c/508061 to remove
tools/tox_install.sh.

Fixes requirements install failure with stable jobs.

Change-Id: Ie2bf2618c8e202653a45cde195bd395ab7058dc1
(cherry picked from commit f999996e)