LU-6081 lfs: split setstripe and migrate help

LU-6081 lfs: split setstripe and migrate help

The setstripe and migrate share all but one of their parameters
(--block). However the help is common to both, so the --block also
appears for the setstripe command but is not valid. So split the help
message for each command, while still sharing the same text to avoid
duplication.

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: If8d110b04b26090d0540bb1628de75a94d92727e
Reviewed-on: http://review.whamcloud.com/13317
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>