Add DatasetNamespace.contains()

Needed for https://issues.cask.co/browse/CDAP-1046
By default for user namespace we have `namespacePrefix = "cdap"` and then namespace is `Namespace.USER`.


For a call to `contains("cdap.user.something"`, we would do

```
name.startsWith("cdap") &&
Namespace.USER.contains("user.something") // will do "user".startsWith(prefix)
```