Always use upper-constraints

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

Change-Id: I113dd658afebe843db304be6cbc4b1ec5df5d248

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} {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -40,9 +40,13 @@ commands =
     python tools/custom_guidelines.py --exclude heat/engine/resources/aws
 
 [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}'
 
@@ -92,6 +96,8 @@ local-check-factory = heat.hacking.checks.factory
 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
 
 [testenv:install-guide]