Shrink devref.unit_tests, since info is in wiki.

Don't keep a copy of the run_tests.sh flag list here since it
inevitably will get out of sync with the real list.

No need to go into detail on how unit testing is set up, since we already have a wiki
on how to run tests that applies for all OpenStack projects. Keep
section with tox examples since that is the most commonly used command.

Change-Id: Ia8eb059be164d9df3edfd69aedcebfd028da45ff
Related-Bug: 1219040
Merge "Shrink devref.unit_tests, since info is in wiki."