Add the ability to set any mode/lb on bonds

This requires a schema change, adding 'mode' and 'load-balance' fields,
a semantic invariant that 'load-balance' can only be set in the case of
LACP and XOR bonds, a mapper from the mode/lb strings, ie.
"round-robin" to their VPP numeric counterparts, a bunch of unit tests.

Then in the reconciler, changing bonds (__bond_has_diff()) will
invalidate any LCP or sub-interfaces built on them, so those will have
to be pruned. create_bondethernet() will now create (or re-create)
the bond with the correct flags.

Unit-tests, YAML tests and the integration test all pass.

Updated config-guide.

unittest/yaml/error-bondethernet8.yaml

@@ -0,0 +1,44 @@
+test:
+  description: "BondEthernet can only have loadbalance if XOR or LACP"
+  errors:
+    expected: 
+     - "bondethernet BondEthernet2 can only have load-balance if in mode XOR or LACP"
+    count: 1
+---
+bondethernets:
+  BondEthernet0:
+    interfaces: [ GigabitEthernet1/0/0, GigabitEthernet1/0/1 ]
+    mode: xor
+    load-balance: l34
+
+  BondEthernet1:
+    interfaces: [ GigabitEthernet2/0/0, GigabitEthernet2/0/1 ]
+    mode: lacp
+    load-balance: l34
+
+  BondEthernet2:
+    interfaces: [ GigabitEthernet3/0/0, GigabitEthernet3/0/1 ]
+    mode: round-robin
+    load-balance: l34
+
+interfaces:
+  GigabitEthernet1/0/0:
+    mtu: 3000
+  GigabitEthernet1/0/1:
+    mtu: 3000
+  BondEthernet0:
+    mtu: 3000
+
+  GigabitEthernet2/0/0:
+    mtu: 3000
+  GigabitEthernet2/0/1:
+    mtu: 3000
+  BondEthernet1:
+    mtu: 3000
+
+  GigabitEthernet3/0/0:
+    mtu: 3000
+  GigabitEthernet3/0/1:
+    mtu: 3000
+  BondEthernet2:
+    mtu: 3000