Cleanup tox.ini, enable constraints
Cleanup tox.ini, enable constraints

Remove old and unused constraints environments from tox.ini. Those
have never been used. Use standard environments as default list.

Enable use of constraints for all tox based jobs.

For more information about constraints see:
http://lists.openstack.org/pipermail/openstack-dev/2016-August/101474.html

Change-Id: I678b108bbd9ca2527b4f9090bb8ef1e84977dee2

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun