CDAP-772 Feature/cdap 772 namespace service

Namespaces APIs to manage namespaces metadata.
Question: 
Should we create and use ``Id.Namespace`` instead of using namespace name as ``Key``?

- [x] Namespace service to create, list, delete namespaces
- [x] Unit testing, standalone testing
- [x] Testing on a distributed cluster. 
why executeUnchecked()? This method can throw Exception, so no need to wrap exceptions into RuntimeExceptions with Throwables.propagate().