Add int_to_mode() and int_to_lb() in config/bondethernet.py to map back
the retrieved values from VPP into their config strings.
Implement bond and bridge settings dumper, dumping all settings even if
they are default. This helps the user understand the configurable
options.
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.
