Add support for loading lists of tempest tests

Add support for the tempest run --load-list feature, along with an extensive example how it can be used.

One caveat: This only runs upstream tempest as-as, so in my experience there is no guarantee that the specified list of tests will actually be executed. If there is anything missing in the tempest conf, it will just ignore the requested tests affected.

Motivation: Due to the OpenStack interop group being retired, their tooling "refstack(-client)" has also been abandoned. SCS-IaaS requires OpenStack Powered Compute compliance, and that set of tests is planned to be defined in an SCS standard now to remove the dependceny on refstack. It hasn't been formalized but IMHO there is a consensus that the load-list format will be how required tempest tests are going to be documented.