ipatests: move ipa_backup to tasks
    
    * tasks had an ipa_backup() method that was not used anywhere.
    * test_backup_and_restore had a backup() method that used to return
      both the path to the backup and the whole result from run_command ;
      The path to the backup can be determined from the result.
    
    Clean up:
    * move test_backup_and_restore.backup to tasks.ipa_backup, replacing
      the unused method.
    * add tasks.get_backup_dir(host) which runs ipa-backup on host and
      returns the path to the backup directory.
    * adjust test_backup_and_restore and test_replica_promotion.
    
    Related: https://pagure.io/freeipa/issue/8217
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>