Always use upper-constraints

This patch modify tox.ini to always use upper-constraints.

Change-Id: I113dd658afebe843db304be6cbc4b1ec5df5d248
(cherry picked from commit 2e6ce70695)
(cherry picked from commit 12e31c8a26)

tox.ini

@@ -6,7 +6,7 @@ skipsdist = True
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
-install_command = pip install {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/liberty} {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -39,9 +39,13 @@ commands =
     bash -c "find heat -type f -regex '.*\.pot?' -print0|xargs -0 -n 1 msgfmt --check-format -o /dev/null"
 
 [testenv:venv]
+# Note: infra doesn't support to use of constraints for this target.
+install_command = pip install -U --force-reinstall {opts} {packages}
 commands = {posargs}
 
 [testenv:cover]
+# Note: infra doesn't support to use of constraints for this target.
+install_command = pip install -U --force-reinstall {opts} {packages}
 commands =
   python setup.py testr --coverage --testr-args='^(?!heat_integrationtests){posargs}'
 
@@ -83,4 +87,6 @@ import_exceptions = heat.common.i18n
 commands = oslo_debug_helper {posargs}
 
 [testenv:releasenotes]
+# Note: infra doesn't support to use of constraints for this target.
+install_command = pip install -U --force-reinstall {opts} {packages}
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html