if any sub-int exists with a certain [dot1q,dot1ad,inner-dot1q], no
other sub-int in the same parent may exist with the same encapsulation.
- refactor sub-int mtu check to use get_mtu()
- if a sub-int has no config, raise a validation error
Refactor validator main function to avoid 'interface' symbol clash.
Add get_mtu() for interfaces, returns the sub-int's MTU or its parent's MTU, defaulting to 1500.
Ensure MTU for all bridge-domain members is set to the same value.
Ensure all bridge-domain members are L2 (have no LCP, have no address)
