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/test_bondethernet.yaml

@@ -2,6 +2,15 @@ bondethernets:
   BondEthernet0:
     interfaces: [ GigabitEthernet1/0/0, GigabitEthernet1/0/1 ]
 
+  BondEthernet1:
+    interfaces: [ GigabitEthernet3/0/0, GigabitEthernet3/0/1 ]
+    mode: xor
+    load-balance: l2
+
+  BondEthernet2:
+    interfaces: [ GigabitEthernet4/0/0, GigabitEthernet4/0/1 ]
+    mode: round-robin
+
 interfaces:
   GigabitEthernet1/0/0:
     mtu: 3000
@@ -14,6 +23,16 @@ interfaces:
       100:
         mtu: 2000
 
+  GigabitEthernet3/0/0:
+    mtu: 3000
+  GigabitEthernet3/0/1:
+    mtu: 3000
+
+  GigabitEthernet4/0/0:
+    mtu: 3000
+  GigabitEthernet4/0/1:
+    mtu: 3000
+
   BondEthernet0:
     mtu: 3000
     lcp: "be012345678"
@@ -22,3 +41,9 @@ interfaces:
       100:
         mtu: 2000
         addresses: [ 192.0.2.9/29, 2001:db8:1::1/64 ]
+
+  BondEthernet1:
+    mtu: 3000
+
+  BondEthernet2:
+    mtu: 3000