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
Lustre-change: https://review.whamcloud.com/29776
Lustre-commit: bac71754b9dbb83b3ccf10ae99c027cc6b2f9829

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I0d662e9e24bed17078e31b6b5ca305cff2c0548e
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/30273
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>