Test and debug interface improvements

Added functionality:
- Get all E2Nodes in subscription manager
- Get all REST subscriptions of one E2Node in Subscription manager
- Get all xApps in subscription manager
- Get all subscriptions of a xApp in Subscription manager
- Get all E2 subscriptions of a REST subscription
- Delete all subscriptions of one E2Node
- Delete all subscriptions of a xApp

To be done:
- Document update not yet done
- Testing in real environment not yet done

Change-Id: I5e270c27efa4f8b2da6283540d5fa51aa6146387
Signed-off-by: Anssi Mannila <anssi.mannila@nokia.com>