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: I29e2f542de8358908bded2e843adaeb4b1c12ace
1. Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
2. Update some URLs to latest
Change-Id: Ib934cd24f4af8440458f0d24df90b77e0a3040b5
Lukáš Piwowarski