LU-9480 lnet: tune lnet_peer_discovery_disabled with lnetctl

LU-9480 lnet: tune lnet_peer_discovery_disabled with lnetctl

A new tunable, lnet_peer_discovery_disabled, has been introduced.
Make it tunable with lnetctl. Note that the state of discovery is
reported as 1/enabled, or 0/disabled, which is the inverse of the
module parameter.

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: I67333d86520c5b6db8ff99c924054c4b487c8029
Reviewed-on: https://review.whamcloud.com/25787
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>