Fixes LiveBlockMigrationTestJSON.test_iscsi_volume bad call

_migrate_server_to is being called with 2 arguments instead of 3.

Adds default value for volume_backed argument.

Closes-Bug #1552725

Change-Id: I771de7d28f227460cd9988bff23283a63ee87186
diff --git a/tempest/api/compute/admin/test_live_migration.py b/tempest/api/compute/admin/test_live_migration.py
index 653a3cd..db4ec89 100644
--- a/tempest/api/compute/admin/test_live_migration.py
+++ b/tempest/api/compute/admin/test_live_migration.py
@@ -62,7 +62,7 @@
     def _get_host_for_server(self, server_id):
         return self._get_server_details(server_id)[self._host_key]
 
-    def _migrate_server_to(self, server_id, dest_host, volume_backed):
+    def _migrate_server_to(self, server_id, dest_host, volume_backed=False):
         block_migration = (CONF.compute_feature_enabled.
                            block_migration_for_live_migration and
                            not volume_backed)