Meta data service improvement with code refactor

Fixes for STRATOS-998 and STRATOS-999

This PR contains- 

1. Remove the extend of AbstractAdmin class and get the registry from OSGI service
2. getDataRegistryFactory - change method name as getDataStore
3. Remove TopologyListener as we are using the Rest API Service for delete meta data.

STRATOS-999 - In master branch build, meta data service is not bundle with the Stratos pack.

Please review and merge
Consider using a Constant.