[ci] Launch tox at real requirements

Rally team abandoned alignment to OpenStack global-requirements. This
decision allows us to use libraries which we want, pin versions before
the rally releases and so on.
Despite the fact that we continue synchronizing with g-r regulary (to be
compatible with openstack clients), some dependencies can differ.

Unit and functional tests are designed to be check Rally itself, so it
makes sence to install the packages which fits our requirements, not
global one.

Change-Id: I6c5306732395ce0caf0a1bcad8fc99c1330c8920

tox.ini

@@ -17,7 +17,7 @@ whitelist_externals = find
                       make
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-install_command = pip install -c https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt -U {opts} {packages}
+install_command = pip install -U {opts} {packages}
 usedevelop = True
 commands =
   find . -type f -name "*.pyc" -delete