Show progress output while running unit tests

This patch imports some magic to enable showing test output as the
tests run.  Seeing output showing tests as they execute is reassuring
that the test run is continuing to make progress.  This originated in
tempest and was then brought over to Nova in this commit:

  https://review.openstack.org/#/c/121914/

Note the justification in that commit to just copy it instead of move
it to a shared repo.

To get the old behavior where you only get output when something
fails, you can set the TRACE_FAILONLY env var.  That originated in
this commit in Nova:

  https://review.openstack.org/#/c/123801/

Change-Id: I6a079ad5f630383650c0f9dee09e04250d0e2c5a
Merge "Show progress output while running unit tests"