Enable QoS scenario tests

This test has been disabled since [1].
But it seems the failure ratio is not so high with
the current code.  Let's re-enable it now and see how it goes.

Note: This same test has been enabled on the gate jobs for
other implementation for a while. It doesn't fail much there
either. (networking-midonet)

[1] I37f1488db258f6a4d383fb472cb5433c65371ac5

Related-Bug: #1662109
Change-Id: Ia39c73189ad8a3331c1911989fe69428f064f7a6

neutron/tests/contrib/gate_hook.sh

@@ -104,8 +104,6 @@ case $VENV in
     load_conf_hook osprofiler
     if [[ "$VENV" =~ "dsvm-scenario" ]]; then
         load_rc_hook ubuntu_image
-        # bug/1662109
-        export DEVSTACK_GATE_TEMPEST_REGEX="^(?!.*(?:neutron\.tests\.tempest\.scenario\.test_qos.*))^neutron\.tests\.tempest\.scenario\."
     fi
     if [[ "$VENV" =~ "dsvm-scenario-linuxbridge" ]]; then
         load_conf_hook iptables_verify