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. 
Agreed. We do not want to expose update API for namespaces right now. We may do it later. I will make this API POST for now and return a CONFLICT if it already exists. We can add a PUT API for update later as needed. Is that ok?