Use ostestr instead of the custom pretty_tox.sh

Switch to os-testr as wrapper for testr, instead of the custom
script pretty_tox.sh.

Change-Id: If10ac96ac32813714be2b83f895071820126dbd8

tools/pretty_tox.sh

@@ -1,12 +0,0 @@
-#!/usr/bin/env bash
-
-set -o pipefail
-
-TESTRARGS=$1
-python setup.py testr --testr-args="--subunit $TESTRARGS" | subunit-trace -f
-retval=$?
-# NOTE(mtreinish) The pipe above would eat the slowest display from pbr's testr
-# wrapper so just manually print the slowest tests.
-echo -e "\nSlowest Tests:\n"
-testr slowest
-exit $retval

tox.ini

@@ -11,7 +11,7 @@ deps = -r{toxinidir}/test-requirements.txt
        .[ldap,memcache,mongodb]
 commands =
   find keystone -type f -name "*.pyc" -delete
-  bash tools/pretty_tox.sh '{posargs}'
+  ostestr '{posargs}'
 whitelist_externals =
   bash
   find