Add bridge-domain support.

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)

validator/loopback.py

@@ -14,7 +14,7 @@ def get_by_name(yaml, ifname):
     return None
 
 
-def loopback(args, yaml):
+def validate_loopbacks(args, yaml):
     result = True
     msgs = []
     logger = logging.getLogger('vppcfg.validator')