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.

Change-Id: I42b7dcd3e9e41688ac3bd8b9c9fb9b39f90c3e9a
2 files changed