Allow bondethernet members list to be empty, and omitted from the YAML config

unittest/yaml/correct-bondethernet.yaml

@@ -1,5 +1,5 @@
 test:
-  description: "An example of well formed bondethernet"
+  description: "An example of well formed bondethernets"
   errors:
     count: 0
 ---
@@ -8,6 +8,10 @@ bondethernets:
     interfaces: [ GigabitEthernet1/0/0, GigabitEthernet1/0/1 ]
   BondEthernet1:
     interfaces: [ GigabitEthernet2/0/0, GigabitEthernet2/0/1 ]
+    mode: xor
+    load-balance: l2
+  BondEthernet2:
+    mode: round-robin
 
 interfaces:
   GigabitEthernet1/0/0:
@@ -35,3 +39,6 @@ interfaces:
         mtu: 2000
         lcp: "be1.2000"
         addresses: [ 192.0.2.9/29, 2001:db8:1::1/64 ]
+
+  BondEthernet2:
+    mtu: 1500