Disable reachability-check of default-route for agents

I faced a couple of network issues, mostly caused by network restarts.
In these cases, the configured default route was not added, and the
logs suggested enabling the GatewayOnLink option. This change actually
does this. Beside that, the dedicated Route-section highlights that
the options belong to the routing.

roles/agent-setup/templates/nic.network.j2

@@ -2,4 +2,6 @@
 Name={{ target_nic }}
 [Network]
 DHCP=yes
+[Route]
 Gateway={{ private_nat }}
+GatewayOnLink=yes