Validation check for DHCP on interfaces
Migrated from: https://tracker.clearos.com/view.php?id=1707
Reported by: Dave Loper
Sometimes DHCP remains active and in effect when interfaces are selected to be External.
During a role change and especially on interfaces marked 'External' a validation should look at the dnsmasq scope and warn the user if they are setting up a DHCP scope on an external interface or if the scope of an existing subnet is invalidated by the change to the network configuration.
Deleting an interface should also delete any DHCP configuration