Add UndercloudLocalMtu parameter

Change-Id: I638aef1cf18137692a40ccf7d81c37fed9ab81d9

net-config-undercloud.j2.yaml

@@ -21,6 +21,10 @@ parameters:
     default: []
     description: A list of DNS servers (2 max for some implementations) that will be added to resolv.conf.
     type: comma_delimited_list
+  UndercloudLocalMtu: # Override this via parameter_defaults
+    default: 1500
+    description: MTU to use for the Undercloud local_interface.
+    type: number
 resources:
   OsNetConfigImpl:
     type: OS::Heat::SoftwareConfig
@@ -52,6 +56,7 @@ resources:
                   name: interface_name
                   # force the MAC address of the bridge to this interface
                   primary: true
+                  mtu: {get_param: UndercloudLocalMtu}
 outputs:
   OS::stack_id:
     description: The OsNetConfigImpl resource.