Add OS-SCH-HNT in extensions list

To have backward compatibility with MV 3.53, we need to include
OS-SCH-HNT in the ``cinder list-extensions`` response.

Change-Id: I9da1af4ac6df1d033a9baaba77135b329c87e6f8
Closes-Bug: #1831623
(cherry picked from commit 9a5bdb308a)

cinder/api/contrib/scheduler_hints.py

@@ -12,6 +12,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from cinder.api import extensions
 from cinder.api.schemas import scheduler_hints
 from cinder.api import validation
 
@@ -31,3 +32,13 @@ def create(req, body):
         _validate_scheduler_hints(req=req, body=scheduler_hints_body)
         body['volume']['scheduler_hints'] = scheduler_hints_body.get(attr)
     return body
+
+
+# NOTE: This class is added to include "OS-SCH-HNT" in the list extensions
+# response and "OS-SCH-HNT" is still not loaded as a standard extension.
+class Scheduler_hints(extensions.ExtensionDescriptor):
+    """Pass arbitrary key/value pairs to the scheduler."""
+
+    name = "SchedulerHints"
+    alias = "OS-SCH-HNT"
+    updated = "2013-04-18T00:00:00+00:00"

cinder/api/extensions.py

@@ -31,7 +31,7 @@ import cinder.policy
 CONF = cfg.CONF
 
 LOG = logging.getLogger(__name__)
-FILES_TO_SKIP = ['resource_common_manage.py', 'scheduler_hints.py']
+FILES_TO_SKIP = ['resource_common_manage.py']
 
 
 class ExtensionDescriptor(object):