Block clearing subnet gateway when attached to a router.
Block clearing subnet gateway when attached to a router.

Change-Id: Id40125ffd9892aac30b3685acfdfcc399f642203
Depends-On: Ib53f0e32b97458521086ae04e38a5c7d0910c56c
Closes-Bug: OPENSTACK-2240

nuagenetworks/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron