LU-18114 snapshot: split "lctl snapshot_*" group into subcommands

LU-18114 snapshot: split "lctl snapshot_*" group into subcommands

Split "lctl snapshot_*" command group into subcommands, e.g.
"lctl snapshot_create" to "lctl snapshot create".
Also, lsnapshot in test-framework.sh is modified to verify
this patch.

Test-Parameters: trivial fstype=zfs testlist=sanity-lsnapshot
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: I30bf6f8a853f57bd1a14d5acb2b5aa4b3d49be71
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56120
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Frederick Dilger <fdilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>