Update the README file to use tox -e all

This patch updates the README file to have instructions for running the tempest tests using the "all" environment instead of "all-plugins". The "all-plugins" environment has been deprecated for years and uses site-packages, which may cause unintended test scenarios.

Change-Id: I297babd2e97ad146f60f32ae9073e47a62973860
1 file changed