LU-3882 hsm: Prevent duplicate CT registrations

LU-3882 hsm: Prevent duplicate CT registrations

Associate copytool registration to a given MDC import so that
multiple mounts of the same filesystem do not lead to having the
copytool registered multiple time.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I5767df50331675a1650dda1ab5fc8440aad2a52e
Reviewed-on: http://review.whamcloud.com/7612
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>