Cinder-backup service is not working with multiple availability zones

Issue

• The cinder-backup service is not working and failing with the error.

  $ openstack volume backup create vtest
  Service cinder-backup could not be found. (HTTP 503) (Request-ID: req-75bcc294-6be6-497c-ab0c-bcaa5d3bf22b)
  

• The cinder service-list shows that the backup service in enabled and running state.

  cinder service-list :
  | Binary           | Host                       | Zone | Status  | State | Updated_at                 | Cluster | Disabled Reason | Backend State |
  +------------------+----------------------------+------+---------+-------+----------------------------+---------+-----------------+---------------+
  | cinder-backup    |   ctl01                    | nova | enabled | up    | 2020-03-27T10:08:58.000000 | -       | -               |               |