Set Standalone's br-ctlplane MTU

Set the br-ctlplane MTU to the InterfaceLocalMtu value. This is
required when multiple Standalone nodes are used in a split-stack
deployment.

Closes-Bug: #1823993
Change-Id: I84a35d529ee67e939db1cdadd103f88295e41e8f

net-config-standalone.j2.yaml

@@ -108,6 +108,7 @@ resources:
               - network_config:
                 - type: ovs_bridge
                   name: br-ctlplane
+                  mtu: {get_param: InterfaceLocalMtu}
                   use_dhcp: false
                   dns_servers:
                     get_param: DnsServers