Fixed JSON schema to make properties of a task optional

[johannes-b]

This PR:

• Changes the JSON schema of Shipyard to make the properties of a task optional and not mandatory
• Sets the type of properties to object.

The examples of a Shipyard show this correctly, but the JSON schema was wrong:

    - name: hardening
      sequences:
        - name: delivery
          triggeredOn:
          - event: dev.delivery.finished
          tasks:
          - name: deployment
            properties:
              deploymentstrategy: blue_green_service
          - name: test
            properties:
              teststrategy: performance
          - name: evaluation                                         < e.g., does not have `properties` 
          - name: release

Signed-off-by: Johannes johannes.braeuer@dynatrace.com

[christian-kreuzberger-dtx]

LGTM