Switch to using subunit-trace from tempest-lib

This commit removes the tempest copy of subunit-trace and uses the
entry-point from tempest-lib instead.

Partially implements bp tempest-library

Change-Id: Ic1aeddc2d0b217c41b6b9914ac3f269b80c4ec8e
diff --git a/tools/pretty_tox.sh b/tools/pretty_tox.sh
index 0a04ce6..ff554c5 100755
--- a/tools/pretty_tox.sh
+++ b/tools/pretty_tox.sh
@@ -3,4 +3,4 @@
 set -o pipefail
 
 TESTRARGS=$1
-python setup.py testr --slowest --testr-args="--subunit $TESTRARGS" | $(dirname $0)/subunit-trace.py --no-failure-debug -f
+python setup.py testr --slowest --testr-args="--subunit $TESTRARGS" | subunit-trace --no-failure-debug -f