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_serial.sh b/tools/pretty_tox_serial.sh
index db70890..e0fca0f 100755
--- a/tools/pretty_tox_serial.sh
+++ b/tools/pretty_tox_serial.sh
@@ -7,7 +7,7 @@
 if [ ! -d .testrepository ]; then
     testr init
 fi
-testr run --subunit $TESTRARGS | $(dirname $0)/subunit-trace.py -f -n
+testr run --subunit $TESTRARGS | subunit-trace -f -n
 retval=$?
 testr slowest