Both 'consul config write' and server bootstrap config entries take a
decoding detour through mapstructure on the way from HCL to an actual
struct. They both may take in snake_case or CamelCase (for consistency)
so need very similar handling.
Unfortunately since they are operating on mirror universes of structs
(api.* vs structs.*) the code cannot be identitical, so try to share the
kind-configuration and duplicate the rest for now.
