need bootstrap magic value to delete property

```
Something like

property.key=${bootstrap.DELETE}

to cause property.key not to be present in the final published map.

Use case: java.naming.initial.context has different behaviour depending on 
null/empty string/real value. Need to be able to delete it entirely.
```

Original issue reported on code.google.com by `joe.j.kearney` on 13 Jun 2013 at 4:30
