CPS-1401 Implement V2 of GET Data Node API

CPS-1401 Implement V2 of GET Data Node API

- Modified the GET Data Node API so it returns all the data nodes when
  xpath set to root "/"
- Fragment Repository now returns a collection of Fragment Entities
- Instead of returning only the first Fragment Entity now all fragment
  entities are returned when xpath is set to root
- The Fragemnt Entities are further processed to a Collection of Data
  Nodes. As opposed to singular Data Node in current implementation.
- Finally the DataRestController also returns a Collection of Data
  Nodes when xpath is set to root and valid data is present
- Response body changed from JSON object to JSON Array.
- Exception handling for invalid xpath and non-existing xpath is now
  done separately at persistence layer.
- Refactored code against CPS-1422
- Deprecated getDataNode method from Service and Persistence layer
- Modified V1 of Get Data Node API to use the getDataNodes method and
  get the first data node from the collection returned.
- Modified NCMP to use getDataNodes method
- NCMP still does not support multiple data nodes. It retrieves the
  first data node from the collection returned by getDataNodes

Signed-off-by: arpitsingh <as00745003@techmahindra.com>
Change-Id: I494a5740a53f65376d135fcb9f1e2e8900a2803e