LU-11475 lnet: allow deleting router primary_nid

LU-11475 lnet: allow deleting router primary_nid

Discovery doesn't allow deleting a primary_nid of a peer. This
is necessary because upper layers only know to reach the peer by
using the primary_nid. For routers this is not the case. So
if a router changes its interfaces and comes back up again, the
peer_ni should be adjusted.

Test-Parameters: forbuildonly
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I9da056172f35a5f15eed5ba0e02fcb37ac414c54
Reviewed-on: https://review.whamcloud.com/33300
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Tested-by: Jenkins