Make all linting tests use upper-constraints

As per [1] all linting tests can now use upper-constraints. This patch
removes all instances of the install_command override relating to lint
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: I016b1c1f77a7cc9b2020d8455208a977cb80ffc6

tox.ini

@@ -42,22 +42,12 @@ extensions = .rst
 allow-long-titles=1
 
 [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}