LU-12423 lnet: honor discovery setting

LU-12423 lnet: honor discovery setting

If discovery is off do not push out any updates. This could be
triggered in case of a gateway's interface changing.

Test-Parameters: trivial
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: Ie421318ae85b895327ec170ffb436c9b679f6866
Reviewed-on: https://review.whamcloud.com/35192
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>