Currently, at least one container needs to be provided to get data under an anchor. 
We are assuming that we only have one instance tree under one anchor.
an empty xpath parameter and a single / should we treated the same.

We also try using the same rest endpoint and java API.
This user story is to allow using just '/' to retrieve a data tree under an anchor. (Multiple data trees will be handled in the future - out of scope)
see cps-rest/src/main/java/org/onap/cps/rest/controller/DataRestController.java
This user story replace a TODO in the aforementioned class

A/C

1. calling a method with / will return the top data node
2. calling a method with empty value will return the top data node
3. If there is no data then return an empty json string
4. Update open API specification