LU-10561 flr: remove "--parent" option from lfs mirror command

LU-10561 flr: remove "--parent" option from lfs mirror command

"--parent" option for "lfs mirror create/extend" command was
originally designed to use default stripe options inherited
from parent directory. However, if parent directory has
composite layout, there will be inconsistency to choose the
stripe options from which component to inherit. And if there
is any other option specified, it's also inconsistent to
inherit the layout of parent directory.

So, this patch removes "--parent" option to eliminate ambiguity.
For "--pool|-p" option, this patch supports specifying "none" to
clear the pool name and inherit from parent directory.

Unspecified stripe count, stripe size and OST pool name will
inherit from previous component. If there is no previous component,
then unspecified stripe count and stripe size attributes will
inherit from filesystem-wide default values. Unspecified or
cleared OST pool name will inherit from parent directory.

Change-Id: Ib0ec3cbc65fb307c42881f35dc676090ab8319ff
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: https://review.whamcloud.com/31298
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>