LU-14990 tests: Detect correct LNet interface for sanity-lnet

LU-14990 tests: Detect correct LNet interface for sanity-lnet

Determine the names of the interfaces used for LNet by parsing the
NIDs configured after calling load_modules(). Tests which reference
eth0 are modified to use the interface associated with the primary
NID (i.e. first NID output by lctl list_nids).

Lustre-change: https://review.whamcloud.com/44857
Lustre-commit: f9669c4d3092d44cbc2e2d3c225aee6ebaf268e9

Test-Parameters: trivial testlist=sanity-lnet
HPE-bug-id: LUS-10385
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Id715aa3e5470d9c110f6248620b1a83920875e7b
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/45760
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>