confusing error messages with root namespace

There have been a few instances where a bad configuration value shows a misleading error message regarding the key name.

- [root.graph](https://groups.google.com/d/msg/janusgraph-users/JCsnpHMnKeg/8NEnGuyoBwAJ)
- [root.index](https://groups.google.com/d/msg/janusgraph-users/JtJ0JONIyck/aDKt5fM_CwAJ)
- [root.attributes](https://groups.google.com/d/msg/janusgraph-users/dOpxRbi-ZQI/xWYv4T1MAgAJ)

For example:

```
gremlin> mgmt.set('graph.set-vertex-id', true)
Cannot change the fixed configuration option: root.graph.set-vertex-id
```

The actual property that the user would need to set in the properties is `graph.set-vertex-id`. Most of the config options in `GraphDatabaseConfiguration` ultimately extend from the [`ROOT_NS`](https://github.com/JanusGraph/janusgraph/blob/v0.1.1/janusgraph-core/src/main/java/org/janusgraph/graphdb/configuration/GraphDatabaseConfiguration.java#L99) namespace and the `toString()` method includes "root".

Start in janusgraph-core/src/main/java/org/janusgraph/graphdb/configuration/GraphDatabaseConfiguration.java, especially the ROOT_NS definition and configuration toString() behavior. Reproduce the graph.set-vertex-id example and inspect how configuration keys are rendered. Done means errors identify the user-facing property without the misleading root namespace.

Written by the indexing model from the issue text.