LU-2016 mdd: add layout swap between 2 objects

LU-2016 mdd: add layout swap between 2 objects

This patch add a new method in mdd to swap the layouts between
2 lustre objects.
The 2 objects have to be of the same type.

Change-Id: I26dfef2745eac67168aeceac196453c5126148fd
Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-on: http://review.whamcloud.com/4189
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>