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 we do. I do not know how exactly right now, which is why I added another constant for v3 right now. 

I made this v3 per the kick-off meeting. I think the thought was that this particular API may not be backward-incompatible, but the changes that we will introduce in other APIs to make them namespace-aware will be.