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.
Change-Id: I7f06c0be47f57a9c1ee3cadc7328b1b1b138b3cd
(cherry picked from commit aede01a0a8)