LU-13026 lnet: MR selection of gateway ni

LU-13026 lnet: MR selection of gateway ni

Multi-Rail gateways can only have 1 route pointing to them. Use
the MR selection algorithm to get the best lpni on the MR
gateway to use.

Using the selection algorithm to find the gateway NI, allows us
to use the standard MR criteria: health, preference and credits.

Test-parameters: trivial

Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I65526c6b64a5702734949c9a583a7558614ceae2
Reviewed-on: https://review.whamcloud.com/36913
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>