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
- [x] Testing on a distributed cluster. 
If all we can say about the reason for throwing an exception is that it's thrown if a problem occurs, you don't need to have the tag. I'd drop all these.

If you want a tag, it should be written in the past tense, as described here (http://www.liferay.com/community/wiki/-/wiki/Main/Javadoc+Guidelines#section-Javadoc+Guidelines-@throws+tags):

@throws Exception if a problem occurred while creating the namespace