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. 
Yes, this is the first v3 API, and I am unsure how the migration should work at this point. Should all APIs be made v3 going forward? I thought we'll use another constant for this right now, and consolidate when we are ready?