LU-10161 lbuild: fix the problem of determine default_iface

LU-10161 lbuild: fix the problem of determine default_iface

The script of determining the default interface is broken.
Remove complexity and replace with hostname

Test-Parameters: trivial
Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I0d662e9e24bed17078e31b6b5ca305cff2c0548e
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/29776
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>