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. 
Let's favor the right, since we currently mostly use "== null":

``
$ find . -name *.java | xargs grep "== null" | wc -l
     944
$ find . -name *.java | xargs grep "null ==" | wc -l
      11
``