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. 
Is it necessary to create a new table only for the namespaces? 
Other objects in the meta data store are in the same table.... Why not simply add the namespaces to the existing Store? And so we still need the Account meta data in Store?