Integration/riak admin handoff team

WIP: Do not merge
Pretty sure @angrycub will comment on this, but in other areas we tended not to have config keys that include the state of the thing in the name, especially when the type is a flag.  How about "handoff.outbound" -> on/off? It's easy to invert the flag, for example:

```erlang
{mapping, "handoff.outbound", "riak_core.disable_outbound_handoff", [
  {datatype, {enum, [{on, false}, {off, true}]}},
  {default, on},
  hidden
]}.
```