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: I693e4a106f11e0cadac369fbc186e54ffff13d41