LU-3342 mdt: HSM coordinator agent interface

To move data with external storage, HSM coordinator
uses a Copy Tool running on a client named agent.
This patch implements the interface for these agents.

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I58773a7a975458de15d332f2d4b97988f92cd902
Reviewed-on: http://review.whamcloud.com/6534
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>