LU-3829 ldlm: MDT mount fails on MDS w/o MGS on it

LU-3829 ldlm: MDT mount fails on MDS w/o MGS on it

If we specify multiple --mgsnode for a MDT, when we start MDS upon
it while MGS is no the other node, the MGC import connection will
always select the local nid (which is one of the candidate mgsnode)
since it think its the closest connection.

This patch treats further --mgsnode nids as failover nids, so that
multiple import connections are added for the MGC import.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Ifd015a6df47d4285ff5081a9e1e6eabfc405b193
Reviewed-on: http://review.whamcloud.com/7509
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>