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. 
No such plans in the near future. I had added this for two reasons - 
1. Initially I started off with a separate service for namespaces so this was needed. However, we decided to keep namespaces within appfabric for now, so this is irrelevant.
2. To recognize the "namespaces" endpoint. However, I don't feel strongly about this reason. 
I'll remove this block.