Cuttlefish schema for background manager global kill/enable switch

Add cuttlefish schema for the global background manager kill/enable switch.
* 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_core,
     [{use_background_manager,true},
      {enable_consensus,false},
     …
```