Update devstack to run tests both sequentially and in parallel

We will first run our functional tests sequentially to ensure
that the individual tests pass.  Next we will run them in
parallel to ensure that they still run successfully.

Change-Id: I49bf3452000985a5307500b2d38b0dff785611c1

functionaltests/run_tests.sh

@@ -35,8 +35,16 @@ TEMPEST_DIR=${TEMPEST_DIR:-/opt/stack/new/tempest}
 # Install tempest
 pip install -e $TEMPEST_DIR
 
+# run the tests sequentially
+testr init
+testr run --subunit | subunit-trace --no-failure-debug -f
+retval=$?
+testr slowest
+
+# run the tests in parallel
 testr init
 testr run --parallel --subunit | subunit-trace --no-failure-debug -f
 retval=$?
 testr slowest
+
 exit $retval