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. 
Can there be a race condition if the namespace gets created between namespaceExists() and createNamespace()? 
I guess the store should implement an atomic check-and-create that returns an error if it already exists.