Add bridgedomain settings.

Bridges can be created with default settings, with specific settings,
and they can be sync'd at runtime with all of the settings in this
change.

Notably missing are two features:
- unknown unicast flooding into specific interfaces (as opposed to
  on/off on the bridge)
- learn-limit, which does not have an API getter, only a setter.

unittest/test_bridgedomain.yaml

@@ -51,9 +51,13 @@ bridgedomains:
     bvi: loop0
     interfaces: [ GigabitEthernet1/0/0, GigabitEthernet1/0/1, BondEthernet0 ]
   bd11:
-    description: "Bridge Domain 11, with sub-interfaces"
+    description: "Bridge Domain 11, with sub-interfaces and settings"
     mtu: 2000
     interfaces: [ GigabitEthernet2/0/0.100, GigabitEthernet2/0/1.100, BondEthernet0.100 ]
+    settings:
+      mac-age-minutes: 10
+      unicast-flood: False
+      unknown-unicast-flood: False
   bd12:
     description: "Bridge Domain 12, invalid because it has Gi1/0/0 as well"
     mtu: 9000