Simple update for our painfully out of date devref docs.
Just a short refresh on unit_tests, there's a lot that could
and probably should be added there, but this change just gets
rid of the obsolete references to Nose and adds info regarding
tox and how to use it.
Change-Id: Ib1ff174ee9ccb8ecb258558d77202062204dfec3
ef42efe5f1