LU-11641 lnet: handle discovery off

LU-11641 lnet: handle discovery off

When discovery is turned off locally or when the peer either has
discovery off or doesn't support MR at all then degrade discovery
behavior to a standard ping. This will allow routers to continue
using discovery mechanism even if it's turned off.

Test-Parameters: forbuildonly
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I7f0829d37cbff2bf9e41de251efa715fc4c97e5d
Reviewed-on: https://review.whamcloud.com/33620
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Tested-by: Jenkins