LU-11645 tests: fix sanity-sec test 31

LU-11645 tests: fix sanity-sec test 31

In sanity-sec test 31, command to add new LNet network ${NETTYPE}999
may fail if servers have interface names different from the one used
on the client.
So fix the command so that it is run directly on each node.

Test-Parameters: trivial envdefinitions=ONLY=31 testlist=sanity-sec
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ibf9101524d94188b3beae3debe45e2ba151999ca
Reviewed-on: https://review.whamcloud.com/33622
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sonia Sharma <sharmaso@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>