We already have a template 'horizon-non-primary-django-jobs'
to test horizon and plugins with non-primary django versions,
but we still need to update tox.ini in all horizon plugins
whenever we change Django versions used.
This commit prepares per-Django environment in the zuul job.
Per-Django tox environments like py3-{dj111,dj20,dj22} are
no longer needed. It would be a big merit that we will no longer
need to update tox.ini in all horizon plugins.
The downside is that we do not provide a convenient way to
test it locally, but I think it can be covered in the document.
Change-Id: I726b19130ee9e7d06eb33231071c2673cfd3a49f
c5dd1536a0