Enhance deletion efficiency when backup init host

Enhance deletion efficiency when backup init host

Now in backup init host, if the status of backup is 'deleting', cinder
will delete those backups sequentially. So if the deletion process is
slow, the backup service thread will be blocked.

Similar to volume deletion, the threadpool could be used to
offload all the pending backup delete operations.

Add CONF.backup_service_inithost_offload to specify if using threadpool.

DocImpact
Implements: blueprint enhance-deletion-efficiency-when-backup-init-host

Change-Id: If9d9e454cdd48669db6e4b8d96f340bbb6f99fad