LU-15613 utils: add lfs_migrate --cp option

LU-15613 utils: add lfs_migrate --cp option

Since "lfs_migrate" is only copying a single file at a time, there is
no real benefit from using "rsync" as the only copytool.  Add a short
"--cp" option with suitable arguments to complement "--rsync".

Also allow the copy command to be replaced arbitrarily with RSYNC and
RSYNC_OPTS environment variables to be specified by the caller.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I66c664948d5cc5a7baa5a65550abfaa8d73ebbe5
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49727
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>