C-bak configure more than one worker issue

[ZhaoYixin]

C-bak service with multiple workers（e.g. backup_workers=5）.

when multiple backups are created concurrently, one of the workers exited abnormally and the child process was respawned. Because the process_number was not 1, _cleanup_incomplete_backup_operations function was not executed.

As a result, the backup status on this worker could not be changed from creating to error. In the same way, restore has the same problem.

```
# backup/manager.py

    @utils.synchronized('cleanup_incomplete_backups_%s' % SERVICE_PGRP,
                        external=True, delay=0.1)
    def _cleanup_incomplete_backup_operations(self, ctxt):
        # Only the first launched process should do the cleanup, the others
        # have waited on the lock for the first one to finish the cleanup and
        # can now continue with the start process.
        if self._process_number != 1:
            LOG.debug("Process #%s %sskips cleanup.",
                      self._process_number,
                      '(pgid=%s) ' % SERVICE_PGRP if SERVICE_PGRP else '')
            return

        LOG.info("Cleaning up incomplete backup operations.")
        ......
```

[Sofia Enriquez (lsofia-enriquez)]

Greetings,
Do you mind adding more details so we can reproduce:
- cinder version
- backup driver
Thanks in advance,
Sofia