Make sure debian net post-up never returns an error code.

Inspired by e7dce43af3.

I'm using this configuration in my Vagrantfile:
config.vm.network :public_network, use_dhcp_assigned_default_route: true

Every once in a while the secondary network interface would not come
up properly and cause the init scripts to wait for ~ 3 minutes before
giving up and continue with booting.

templates/guests/debian/network_dhcp.erb

@@ -6,7 +6,7 @@ iface eth<%= options[:interface] %> inet dhcp
     post-up route del default dev $IFACE || true
 <% else %>
     # We need to disable eth0, see GH-2648
-    post-up route del default dev eth0
+    post-up route del default dev eth0 || true
     post-up dhclient $IFACE
     pre-down route add default dev eth0
 <% end %>