The 'all-plugin' tox environment was deprecated by this patch [1].
Instead of the 'all-plugin' it is recommended to use the 'all' tox
environment.
This patch removes any reference to 'all-plugin' tox environment and
updates the documentation so that the installation steps work with
the 'all' venv.
[1] https://review.opendev.org/c/openstack/tempest/+/543974
Change-Id: Ic9c5cc65611c12211be35d9d65c75d67ab717302