Fix three resources not being deleted by using name

There are three resources: VolumeBackup, Snapshot, VolumeTransfer.
None of them can be deleted by using their names on V2.

For example, Snapshot client side filtering fails in the function
findall. The reason is that snapshot object
<class 'cinderclient.v2.volume_snapshots.Snapshot'> does not have
a attribute 'display_name'.

The response of /v2/{tenant_id}/backups/detail does not have
a parameter 'display_name', but have a parameter 'name'.
So class Snapshot does not need to define NAME_ATTR = 'display_name'.
The superclass of Snapshot 'Resource' has defined NAME_ATTR = 'name'.

Closes-Bug: #1497873
Closes-Bug: #1495428
Change-Id: I3c89fd9a82918e8a02385cfccb32dc9c3f4c4095

cinderclient/v2/volume_backups.py

@@ -22,7 +22,6 @@ from cinderclient import base
 
 class VolumeBackup(base.Resource):
     """A volume backup is a block level backup of a volume."""
-    NAME_ATTR = "display_name"
 
     def __repr__(self):
         return "<VolumeBackup: %s>" % self.id

cinderclient/v2/volume_snapshots.py

@@ -26,7 +26,6 @@ from cinderclient import base
 
 class Snapshot(base.Resource):
     """A Snapshot is a point-in-time snapshot of an openstack volume."""
-    NAME_ATTR = "display_name"
 
     def __repr__(self):
         return "<Snapshot: %s>" % self.id

cinderclient/v2/volume_transfers.py

@@ -27,7 +27,6 @@ from cinderclient import base
 
 class VolumeTransfer(base.Resource):
     """Transfer a volume from one tenant to another"""
-    NAME_ATTR = "display_name"
 
     def __repr__(self):
         return "<VolumeTransfer: %s>" % self.id