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. 
Hmmm, I agree. However, I think then it makes sense to revisit Account usage and remove Account from MDS while we're adding namespace to the various entities.  I'll add that to the requirements while making those changes to streams, apps etc.