CDAP-772 Feature/cdap 772 namespace service

Namespaces APIs to manage namespaces metadata.
Questions: 
1. Should any of the ``NamespaceMetaStore`` APIs throw Exceptions at all? 
2. 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.