LU-6868 mdd: add changelog for migration

LU-6868 mdd: add changelog for migration

Add changelog for migration, so robinhood policy engine
can handle the migration command.

Add test_160d to verify the migration changelog

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Iaa33dee607fcd79285f59bd3131d70b7e5329622
Reviewed-on: http://review.whamcloud.com/16645
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>