LU-3929 lwp: don't connect LWP to old MDT

LU-3929 lwp: don't connect LWP to old MDT

When rolling upgrade Lustre from old version without LWP support
(< 2.3.60) to newer version, the OSTs will be upgraded firstly,
then LWP will try to connect the old MDT, and old MDT will regard
the LWP client as normal client and save client export on disk for
recovery, which will break recovery at the end since LWP
connection isn't recoverable.

Change-Id: Iad6de8f8a59ff9606c30dd8b750e7638b6bc4e9c
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-on: http://review.whamcloud.com/8513
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>