Fix ServerMigrationSampleJsonTests to use sample files from version dir

The ServerMigrationSampleJsonTests functional tests set up
``self.api.microversion`` to 2.22 in its' setUp(), which use
sample files from non version dir.

For consistency, this commit makes these tests to use sample
files from versioned dir.

Change-Id: I9af2348cf7dba53e0fef4054bcedcd5b0c626f50

api-ref/source/server-migrations.inc

@@ -167,7 +167,7 @@ Request
 
 **Example Force Migration Complete (force_complete Action)**
 
-.. literalinclude:: ../../doc/api_samples/server-migrations/force_complete.json
+.. literalinclude:: ../../doc/api_samples/server-migrations/v2.22/force_complete.json
    :language: javascript
 
 Response

nova/tests/functional/api_sample_tests/test_server_migrations.py

@@ -29,12 +29,12 @@ from nova.tests.unit import fake_instance
 class ServerMigrationsSampleJsonTest(test_servers.ServersSampleBase):
     sample_dir = 'server-migrations'
     scenarios = [('v2_22', {'api_major_version': 'v2.1'})]
+    microversion = '2.22'
 
     def setUp(self):
         """setUp method for server usage."""
         super(ServerMigrationsSampleJsonTest, self).setUp()
         self.uuid = self._post_server()
-        self.api.microversion = '2.22'
 
     @mock.patch.object(conductor_manager.ComputeTaskManager, '_live_migrate')
     @mock.patch.object(db, 'service_get_by_compute_host')