Use upper constraints for all jobs in tox.ini

Openstack infra now supports upper constraints for
all jobs. Updated tox.ini to use upper constraints
for all jobs.

Change-Id: I1f31ac2761e793a7831676de520de08497e2fb95
Closes-Bug: #1614361

tox.ini

@@ -18,8 +18,6 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:cover]
-# TODO(ihrachys): remove once infra supports constraints for this target
-install_command = pip install -U {opts} {packages}
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [testenv:debug]
@@ -47,8 +45,6 @@ commands =
     python setup.py build_sphinx
 
 [testenv:venv]
-# TODO(ihrachys): remove once infra supports constraints for this target
-install_command = pip install -U {opts} {packages}
 commands = {posargs}
 
 [testenv:genconfig]
@@ -85,6 +81,4 @@ local-check-factory = octavia.hacking.checks.factory
 max-line-length = 79
 
 [testenv:releasenotes]
-# TODO(ihrachys): remove once infra supports constraints for this target
-install_command = pip install -U {opts} {packages}
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html