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 keeps the status quo.
For more information about constraints see:
http://lists.openstack.org/pipermail/openstack-dev/2016-August/101474.html
Change-Id: Iade5865825a022cb4802820bfc564433bb976d06
Andreas Jaeger