Cuttlefish schema for background manager subsystem (aae and handoff) kill/enable switch

Add cuttlefish schema for the KV's background manager "use/kill/enable" switch. When set to "false", KV won't use the background manager for the specified subsystems, namely AAE tree rebuilding and Handoff.
* On by default
* Advanced mode setting means it doesn't get generated into the config file "riak.config"
* Look for it in "dev/devN/data/generated.configs" where N=1..6 after ```make devrel```

I examined the ```generated.configs``` file to confirm it was present and had the correct default value… you should see this:

```
...
 {riak_kv,
     [{aae_use_background_manager,true},
      {handoff_use_background_manager,true},
      {max_siblings,100},
     …
```