Use constraints for all tox environments
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

Hybrid-Cloud/cinder,j-griffith/cinder,cloudbase/cinder,NetApp/cinder,Nexenta/cinder,mahak/cinder,cloudbase/cinder,NetApp/cinder,eharney/cinder,Nexenta/cinder,Datera/cinder,eharney/cinder,phenoxim/cinder,ge0rgi/cinder,mahak/cinder,Hybrid-Cloud/cinder,openstack/cinder,phenoxim/cinder,openstack/cinder,j-griffith/cinder,Datera/cinder