LU-3711 mdt: default archive id is tunable

LU-3711 mdt: default archive id is tunable

Add a /proc entry to specify default archive id. The
default is used when no archive id or 0 archive id are
specified from the command line.

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I0d59c37e7bc9a552d1787cf85d03328989ba9b19
Reviewed-on: http://review.whamcloud.com/7373
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Malcolm Cowe <malcolm.j.cowe@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>