Allow for bridgedomains without any member interfaces

schema.yaml

@@ -15,7 +15,7 @@ bridgedomain:
   mtu: int(min=128,max=9216,required=False)
   lcp: str(max=15,matches='[a-z]+[a-z0-9-]{,7}',required=False)
   addresses: list(ip_interface(),min=1,max=6,required=False)
-  interfaces: list(str())
+  interfaces: list(str(),required=False)
 ---
 loopback:
   description: str(exclude='\'"',len=64,required=False)

unittest/yaml/correct-bridgedomain.yaml

@@ -56,3 +56,5 @@ bridgedomains:
     mtu: 9000
     interfaces: [ GigabitEthernet4/0/0, GigabitEthernet4/0/1 ]
     lcp: "bvi12"
+  bd13:
+    description: "Bridge Domain 13, it is OK to have no member interfaces"