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: I02c57a8eeaf9540e4b94882a581b89533a129350

tox.ini

@@ -5,14 +5,15 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command =
-    {toxinidir}/tools/tox_install.sh {env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_NOCAPTURE=False
          OS_STDERR_NOCAPTURE=False
          PYTHONHASHSEED=0
 
-deps = -r{toxinidir}/requirements.txt
+deps =
+       -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --testr-args='{posargs}'