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

tox.ini

@@ -29,7 +29,7 @@ commands = python setup.py testr --coverage --testr-arg='tempest\.tests {posargs
 sitepackages = True
 setenv = VIRTUAL_ENV={envdir}
 commands =
-  python setup.py testr --slowest --testr-args='{posargs}'
+  bash tools/pretty_tox.sh '{posargs}'
 
 [testenv:full]
 sitepackages = True