Remove old and unused constraints environments from tox.ini. Those
have never been used. Use standard environments as default list.
Note that the repo has in the past not used constraints in OpenStack
CI, this change does not change any of that.
For more information about constraints see:
http://lists.openstack.org/pipermail/openstack-dev/2016-August/101474.html
Change-Id: I5d7230ba31f3585ea3871c11351db31b089742af