LU-12427 lnet: warn if discovery is off

Output a warning if discovery is off and admin is
either trying to add a route or enable routing

Test-Parameters: trivial
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: Iacd7762c5d19c6e0c45ff6a58693a05761f1336f
Reviewed-on: https://review.whamcloud.com/35200
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>