bring over pretty_tox.sh from tempest

Tempest developed a set of wrappers around testr to print out tests as
they are running through using the subunit stream. This also displays
which workers the various tests are running on. Adapted for nova's
need for loading the locking.

I expect that the needs of nova will diverge this over time,
especially what's displayed during tracing, as such I think it's fine
to just do a transplant and not try to turn this into some external
repo.

Change-Id: Ied1708515fc49f4e18cd8dbb7b2cd821f85e1966
Merge "bring over pretty_tox.sh from tempest"