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
shade is in global requirements and upper-constraints now, which means
we need to use the tox_install.sh script so that constraints don't break
install of shade itself.