LU-6032 obdclass: new wrapper to convert NID to string

LU-6032 obdclass: new wrapper to convert NID to string

This patch includes a couple of changes:
- add new wrapper function obd_import_nid2str
- use obd_import_nid2str and obd_export_nid2str to replace all
  libcfs_nid2str conversions for NID of export/import connection

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Change-Id: I57d08e6ef902c6a34c705663de0ed73bb3dc76f2
Reviewed-on: https://review.whamcloud.com/12956
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>