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.

schema.yaml

@@ -35,6 +35,8 @@ loopback:
 bondethernet:
   description: str(exclude='\'"',len=64,required=False)
   interfaces: list(str(matches='.*GigabitEthernet[0-9]+/[0-9]+/[0-9]+'))
+  mode: enum('round-robin','active-backup','broadcast','lacp','xor',required=False)
+  load-balance: enum('l2','l23','l34',required=False)
 ---
 interface:
   description: str(exclude='\'"',len=64,required=False)