Always allow to create snapshot db record with snapshot-manage

Now, Cinder will raise 400 error when users manage a snapshot if
the volume is in-use. We should allow this action in this situation.
So set the "force=True" here when create the snapshot db record.

Change-Id: Ic85ca0ec35a1711ecad6f08432b301ec1f82d6f2
Closes-bug: #1534350

cinder/volume/api.py

@@ -1616,7 +1616,7 @@ class API(base.Base):
                                  metadata=None):
         service = self._get_service_by_host(context, volume.host, 'snapshot')
         snapshot_object = self.create_snapshot_in_db(context, volume, name,
-                                                     description, False,
+                                                     description, True,
                                                      metadata, None,
                                                      commit_quota=False)
         self.volume_rpcapi.manage_existing_snapshot(context, snapshot_object,