More usage of ostestr and cleanup an unused dependency

Use ostestr as runner instead of the custome pretty_tox.sh script
(pretty much removed everywhere). pretty_tox3.sh is just a wrapper
for ostestr but it could be probably removed as well going forward.
Also remove the tempest-lib dependency, which originally provided
subunit-trace (now from os-testr).

Change-Id: I4390964ce83167d634b80f696e82a0e199582ce6

tools/pretty_tox.sh

@@ -1,16 +0,0 @@
-#!/usr/bin/env bash
-
-set -o pipefail
-
-TESTRARGS=$1
-
-# --until-failure is not compatible with --subunit see:
-#
-# https://bugs.launchpad.net/testrepository/+bug/1411804
-#
-# this work around exists until that is addressed
-if [[ "$TESTARGS" =~ "until-failure" ]]; then
-    python setup.py testr --slowest --testr-args="$TESTRARGS"
-else
-    python setup.py testr --slowest --testr-args="--subunit $TESTRARGS" | subunit-trace -f
-fi