Use constraints for all tox environments

Infra is now ready to use constraints everywhere [1], so adapt
releasenotes, coverage, and venv to use the default install command
instead of a specific non-constrained install command.

With this change, all tox environments now use constraints.

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

Change-Id: I879f34129c0d958be0a73bfdc7f641e178d4efe9
(cherry picked from commit 66023c5)

tox.ini

@@ -26,10 +26,6 @@ whitelist_externals = bash
 passenv = *_proxy *_PROXY
 
 [testenv:releasenotes]
-# 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 = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:functional]
@@ -59,10 +55,6 @@ commands = bash tools/lintstack.sh
 [testenv:cover]
 # Also do not run test_coverage_ext tests while gathering coverage as those
 # tests conflict with coverage.
-# 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 =
   python setup.py testr --coverage \
     --testr-args='^(?!.*test.*coverage).*$'
@@ -78,10 +70,6 @@ envdir = {toxworkdir}/pep8
 commands = {toxinidir}/tools/config/generate_sample.sh from_tox --nosamplefile
 
 [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}
 
 [testenv:docs]