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.

intest/hippo10.yaml

@@ -1,6 +1,8 @@
 bondethernets:
   BondEthernet0:
     interfaces: [ GigabitEthernet3/0/0, GigabitEthernet3/0/1 ]
+    mode: lacp
+    load-balance: l2
 
 interfaces:
   GigabitEthernet3/0/0: