LU-17182 utils: pool_add send OSTs in one batch

LU-17182 utils: pool_add send OSTs in one batch

'lctl pool_add' command sends all requests in one batch
then checks results. In this way, the command won't take
too long time if the OSTs are specified in command line
one by one.

Lustre-change: https://review.whamcloud.com/52654
Lustre-commit: TBD (from 7a9b37c84921687ab8ac0c765aab6b5a6b339468)

Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Change-Id: Ibd6e7ed5104e100d44c5f4288a25e7378cd9cfe8
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52719
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>