Fix list_migration response schema for None values

list migration response schema in Tempest does not allow
many of expected nullable attribute as None.

For example dest_host, source_node, dest_node can be None
for "evacuation", "resize" migration type. or in other migration
type Destination and source are mostly driven by "source_compute"
and "dest_compute" .

There is failure for that in mulitnode job -
http://logs.openstack.org/80/202380/2/check/gate-tempest-dsvm-multinode-full/623ac01/console.html

This commit fix list_migration schema as per Nova return response.

Change-Id: I5e81dadd5dee4ecd85f309c34dae0d3ba0733a0d
Closes-Bug: 1475454
diff --git a/tempest/api_schema/response/compute/v2_1/migrations.py b/tempest/api_schema/response/compute/v2_1/migrations.py
index 722372c..b7d66ea 100644
--- a/tempest/api_schema/response/compute/v2_1/migrations.py
+++ b/tempest/api_schema/response/compute/v2_1/migrations.py
@@ -23,15 +23,15 @@
                     'type': 'object',
                     'properties': {
                         'id': {'type': 'integer'},
-                        'status': {'type': 'string'},
-                        'instance_uuid': {'type': 'string'},
-                        'source_node': {'type': 'string'},
-                        'source_compute': {'type': 'string'},
-                        'dest_node': {'type': 'string'},
-                        'dest_compute': {'type': 'string'},
-                        'dest_host': {'type': 'string'},
-                        'old_instance_type_id': {'type': 'integer'},
-                        'new_instance_type_id': {'type': 'integer'},
+                        'status': {'type': ['string', 'null']},
+                        'instance_uuid': {'type': ['string', 'null']},
+                        'source_node': {'type': ['string', 'null']},
+                        'source_compute': {'type': ['string', 'null']},
+                        'dest_node': {'type': ['string', 'null']},
+                        'dest_compute': {'type': ['string', 'null']},
+                        'dest_host': {'type': ['string', 'null']},
+                        'old_instance_type_id': {'type': ['integer', 'null']},
+                        'new_instance_type_id': {'type': ['integer', 'null']},
                         'created_at': {'type': 'string'},
                         'updated_at': {'type': ['string', 'null']}
                     },