LU-14896 utils: migrate file with only '--pool' option

"lfs migrate -p pool_name test_file" initiate a migration but without
changing the layout pools (migrate from layout copy).

This patch implements the same behavior that:
"lfs setstripe -p pool_name test_file"
It sets the pool name and uses the default parameters for the plain
layout.

Add sanity test 56xg to check file migrations with pool.

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: I1645eaca028974337218411d6a033f3acf9b9d6a
Reviewed-on: https://review.whamcloud.com/44465
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>