Make all tests use upper-constraints

As per [1] all tests can now use upper-constraints. This patch removes
all instances of the install_command override relating to testing which
were needed to negate the use of upper-constraints.

[1] http://lists.openstack.org/pipermail/openstack-dev/2016-August/101474.html

Change-Id: I7b6051e9ea0c6e25901ff0fd24bbe5c011c38281

tox.ini

@@ -47,22 +47,12 @@ ignore = D000,D001,D002
 
 
 [testenv:releasenotes]
-# NOTE(sdague): this target does not use constraints because
-# upstream infra does not yet support it. Once that's fixed, we can
-# drop the install_command.
-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
 
 
 # environment used by the -infra templated docs job
 [testenv:venv]
-# NOTE(jaegerandi): this target does not use constraints because
-# upstream infra does not yet support it. Once that's fixed, we can
-# drop the install_command.
-install_command =
-    pip install -U --force-reinstall {opts} {packages}
 commands =
     {posargs}
 
@@ -159,11 +149,6 @@ commands =
 
 
 [testenv:functional]
-# NOTE(odyssey4me): this target does not use constraints because
-# it doesn't work in OpenStack-CI yet. Once that's fixed, we can
-# drop the install_command.
-install_command =
-    pip install -U --force-reinstall {opts} {packages}
 deps =
     {[testenv:ansible]deps}
 setenv =