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.

schema.yaml

@@ -18,6 +18,16 @@ bridgedomain:
   mtu: int(min=128,max=9216,required=False)
   bvi: str(matches='loop[0-9]+',required=False)
   interfaces: list(str(),required=False)
+  settings: include('bridgedomain-settings',required=False)
+---
+bridgedomain-settings:
+  learn: bool(required=False)
+  unicast-flood: bool(required=False)
+  unknown-unicast-flood: bool(required=False)
+  unicast-forward: bool(required=False)
+  arp-termination: bool(required=False)
+  arp-unicast-forward: bool(required=False)
+  mac-age-minutes: int(min=0,max=255,required=False)
 ---
 loopback:
   description: str(exclude='\'"',len=64,required=False)