LU-3828 mdt: hsm tunable can be set permanently

HSM tunable are now always visible, even if hsm is off,
so HSM tunning in MGS conf can be set at MDT mount.
Rename hsm /proc entry archive_id in default_archive_id

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: Ic89de22e11f989ecd2ce01f10764ec6a61a7e870
Reviewed-on: http://review.whamcloud.com/7557
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Malcolm Cowe <malcolm.j.cowe@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>