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: Ib58c11a9d06b78ac9617563ae1baa4844e0efa45
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-on: http://review.whamcloud.com/8701
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>