Choose state 'down' host when scheduler get backup host

[ZhaoYixin]

when I stop all cinder-backup service,

cinder service-list

+------------------+-------------------------+---------+---------+-------+----------------------------+-----------------+
| Binary | Host | Zone | Status | State | Updated_at | Disabled Reason |
+------------------+-------------------------+---------+---------+-------+----------------------------+-----------------+
| cinder-backup | stor01 | nova | enabled | down | 2020-10-26T08:43:36.000000 | - |
| cinder-backup | stor02 | nova | enabled | down | 2020-10-26T08:44:11.000000 | - |
| cinder-backup | stor03 | nova | enabled | down | 2020-10-26T09:32:49.000000 | - |
| cinder-backup | stor04 | nova | enabled | down | 2020-10-26T09:33:04.000000 | - |
| cinder-scheduler | ctl01 | nova | enabled | up | 2020-10-26T09:34:04.000000 | - |
| cinder-volume | stor01@ceph | nova | enabled | up | 2020-10-26T09:34:07.000000 | - |
| ...... | ...... | ...... | ...... |...... | ...... | - |
+------------------+-------------------------+---------+---------+-------+----------------------------+-----------------+

all cinder-backup service state is down.

run command 'cinder backup-create <volume_id> --name backup'

cinder-scheduler still choose one down state cinder-backup host.

########################################

Solution is add to judge whether the service is up.

########################################

def _list_backup_services(self, availability_zone, driver=None):
 """List all enabled backup services.

 :returns: list -- hosts for services that are enabled for backup.
 """
 services = []

 def _is_good_service(cap, driver, az):
  if driver is None and az is None:
   return True
  match_driver = cap['driver_name'] == driver if driver else True
  if match_driver:
   if not az:
    return True
   return cap['availability_zone'] == az
  return False

 for backend, capabilities in self.backup_service_states.items():
  if capabilities['backend_state']:
   if _is_good_service(capabilities, driver, availability_zone):
    services.append(backend)

 return services

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/759786