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
- [ ] Testing on a distributed cluster. 
Actually looks like these are just JSON representations of `NamespaceMetadata`. Please use `NamespaceMetadata` as the type of the constants and use `private int createNamespace(NamespaceMetadata metadata)`, etc. If we want to change NamespaceMetadata later, it will be much easier.