LU-18579 utils: lfs_migrate should inherit OST pool

LU-18579 utils: lfs_migrate should inherit OST pool

Fix the lfs_migrate script so that it inherits the OST pool name
from the source file, if it was specified.

Fix the parsing of the existing file layout so that it can properly
handle various YAML formats.

Test-Parameters: trivial
Fixes: 109b5499d3 ("LU-15565 utils: updated lfs getstripe yaml format")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: If06d3a682e3807e3856f1f5d2cbbecf31c7fa361
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57519
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Sergey Cheremencev <scherementsev@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>