Implement partial threshold / flow deletion logic

[zanetworker]

if a list of thresholds or flows was created, it should be possible detect the removal of an item from the list and also deleting in the monitoring backend. For example if the following was created:

  thresholds:
    - name: "ddos"
      metric: "elephant-flow"
      value: 30
      flowName: "elephant-flow"
    - name: "ddos2"
      metric: "elephant-flow"
      value: 100
      flowName: "elephant-flow"

Editing it to:

  thresholds:
    - name: "ddos2"
      metric: "elephant-flow"
      value: 100
      flowName: "elephant-flow"

should already remove the second threshold from the monitoring backend.