Deleted volumes are being attached to an instance

Issue

• Some volumes are being attached to a instance.

  $ openstack server show aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa
  +-------------------------------------+---------------------------------------------------------------------------+
  | Field                               | Value                                                                     |
  +-------------------------------------+---------------------------------------------------------------------------+
      :
      : ...output omitted
      : 
  | volumes_attached                    | delete_on_termination='False', id='bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb'  |
  |                                     | delete_on_termination='False', id='cccccccc-cccc-cccc-cccc-cccccccccccc'  |
  |                                     | delete_on_termination='False', id='dddddddd-dddd-dddd-dddd-dddddddddddd'  |
  +-------------------------------------+---------------------------------------------------------------------------+
  

• However, some of those volumes don't exist.

  $ openstack volume show cccccccc-cccc-cccc-cccc-cccccccccccc
  No volume with a name or ID of 'cccccccc-cccc-cccc-cccc-cccccccccccc' exists.
  
  $ openstack volume show dddddddd-dddd-dddd-dddd-dddddddddddd
  No volume with a name or ID of 'dddddddd-dddd-dddd-dddd-dddddddddddd' exists.