adds the missing _interface variable

This patch adds the _interface variable in the JSON
test class 'AvailabilityZoneTestJSON'

Change-Id: Ib50626c9e163f13e2cb99b3faa356a17974ab548
Closes-Bug: 1326347
diff --git a/tempest/api/volume/test_availability_zone.py b/tempest/api/volume/test_availability_zone.py
index 1db7b7b..fe8f96e 100644
--- a/tempest/api/volume/test_availability_zone.py
+++ b/tempest/api/volume/test_availability_zone.py
@@ -22,6 +22,7 @@
     """
     Tests Availability Zone API List
     """
+    _interface = 'json'
 
     @classmethod
     def setUpClass(cls):