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. 
similar here: 
```
NameSpaceMeta existing = input.apps.getNamespace(namespaceId);
if (existing != null) {
  input.apps.deleteNamespace(id);
}
return existing;
```