tox.ini started failing with Tox4 which had some
incompatible changes.
Fixing tox.ini for tox4 changes.
The latest version of docker broke kuryr-libnetwork since
it removes external k/v support. We have to hardcode
to an older version of docker to workaround.
The CI failed to delete subnetpool since the subnet haven't
been deleted yet. This commit also fixed it.
Depends-On: https://review.opendev.org/c/openstack/devstack-plugin-container/+/875113
Change-Id: Id48bb7d92f2f5d9d08b44acf651dbba3bf2e924e