Use pretty tox with the ``all`` env

The all entry is usable for running custom set of test
cases selected by a regexp in similar venv
as the gate uses with tox -eall.

The pretty tox helps you to see some basic results before,
 the test suite finished.

Change-Id: I553e450ad0a59308be0bfa24aa66877cdb6667da
Merge "Use pretty tox with the ``all`` env"