FloatingIPQosTest: Stop inheriting QoSTest class

Instead, have a separate class for shared code (QoSTestMixin)
so that we can add test cases to QoSTest without affecting
FloatingIPQosTest.

Change-Id: I2fc2d2a94e78298c2bded00b585c8a1ae852fc0a
diff --git a/neutron_tempest_plugin/scenario/test_floatingip.py b/neutron_tempest_plugin/scenario/test_floatingip.py
index bc40176..0f3d65e 100644
--- a/neutron_tempest_plugin/scenario/test_floatingip.py
+++ b/neutron_tempest_plugin/scenario/test_floatingip.py
@@ -200,7 +200,8 @@
 
 
 class FloatingIPQosTest(FloatingIpTestCasesMixin,
-                        test_qos.QoSTest):
+                        test_qos.QoSTestMixin,
+                        base.BaseTempestTestCase):
 
     same_network = True