Reduce unused tox environments

Since the openstack-ci only tests 5 of the many tox enviroments
we had listed and from my knowledge nobody else is testing using
those other non-standard enviroments just reduce the set of applicable
environments to the ones that are being used (which also means we
can remove the usage of toxgen and just stick with a simpler file
that is easier to use, modify and adjust).

Change-Id: I4d2302594b9f9e8741f9693cb358efc1418bd45d