LU-13090 utils: fix lfs_migrate -p for file with pool

If "lfs_migrate -p <pool>" is run to migrate a file with an existing
pool, the given pool is overridden by the existing pool from the file
during migration.  Fix this to use the OST pool requested by the user.

Don't print a warning about deprecated -n option if --dry-run is used.

If a pool is specified, use it with "lfs df" to find OST free space.

Change temp filename to work better with new DNE "crush" hash.

Don't return an error if falling back to rsync and no links are found.

Add test for "lfs_migrate -p" and update man page and usage to match.
Clean up debug-level helpers in test-framework.sh.

Test-Parameters: trivial testlist=ost-pools
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ief69a620fc969aeff24ec0633a3314c3b83ebbe5
Reviewed-on: https://review.whamcloud.com/37067
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>