Make the lower-constraints tox env actually use lower-constraints

Currently it pulls in upper-constraints from the base testenv definition.

Change-Id: I2f02efb3e5be8b9e497a793b5d0b62fdbe6f6969
Story: #2002900
Task: #22865

tox.ini

@@ -5,14 +5,17 @@ envlist = py3,py27,pep8
 
 [testenv]
 usedevelop = True
-install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          LANGUAGE=en_US
          LC_ALL=en_US.UTF-8
          PYTHONWARNINGS=default::DeprecationWarning
          TESTS_DIR=./ironic/tests/unit/
-deps = -r{toxinidir}/test-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 =
     stestr run {posargs}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY