Simplify xml-json inheritance in compute

_interface just specified in the real test classes.

Change-Id: I40fc6fe4d1a00fa0d83c4343feba73e3192f0991
diff --git a/tempest/tests/compute/test_quotas.py b/tempest/tests/compute/test_quotas.py
index c2ad358..dbff275 100644
--- a/tempest/tests/compute/test_quotas.py
+++ b/tempest/tests/compute/test_quotas.py
@@ -19,10 +19,12 @@
 from tempest.tests.compute import base
 
 
-class QuotasTestBase(object):
+class QuotasTestJSON(base.BaseComputeTest):
+    _interface = 'json'
 
     @classmethod
     def setUpClass(cls):
+        super(QuotasTestJSON, cls).setUpClass()
         cls.client = cls.quotas_client
         cls.admin_client = cls._get_identity_admin_client()
         resp, tenants = cls.admin_client.list_tenants()
@@ -47,17 +49,5 @@
             self.fail("Quota set for tenant did not have default limits")
 
 
-class QuotasTestJSON(QuotasTestBase, base.BaseComputeTestJSON):
-
-    @classmethod
-    def setUpClass(cls):
-        base.BaseComputeTestJSON.setUpClass()
-        super(QuotasTestJSON, cls).setUpClass()
-
-
-class QuotasTestXML(QuotasTestBase, base.BaseComputeTestXML):
-
-    @classmethod
-    def setUpClass(cls):
-        base.BaseComputeTestXML.setUpClass()
-        super(QuotasTestXML, cls).setUpClass()
+class QuotasTestXML(QuotasTestJSON):
+    _interface = 'xml'