Correct volume type api schema validation

Volume type in api response body can be None, so add None value to
volume create and list api  schema validation.

Closes-bug: 1341924

Change-Id: I151b407ea2ec871375cbf96f41269fd8a5414a29
diff --git a/tempest/api_schema/compute/v2/volumes.py b/tempest/api_schema/compute/v2/volumes.py
index 1af951f..541d3ff 100644
--- a/tempest/api_schema/compute/v2/volumes.py
+++ b/tempest/api_schema/compute/v2/volumes.py
@@ -26,7 +26,7 @@
                     'availabilityZone': {'type': 'string'},
                     'createdAt': {'type': 'string'},
                     'displayDescription': {'type': ['string', 'null']},
-                    'volumeType': {'type': 'string'},
+                    'volumeType': {'type': ['string', 'null']},
                     'snapshotId': {'type': ['string', 'null']},
                     'metadata': {'type': 'object'},
                     'size': {'type': 'integer'},
@@ -74,7 +74,7 @@
                         'availabilityZone': {'type': 'string'},
                         'createdAt': {'type': 'string'},
                         'displayDescription': {'type': ['string', 'null']},
-                        'volumeType': {'type': 'string'},
+                        'volumeType': {'type': ['string', 'null']},
                         'snapshotId': {'type': ['string', 'null']},
                         'metadata': {'type': 'object'},
                         'size': {'type': 'integer'},