DCAE APP simulator rework

- Extract message parameters parsing logic to standalone class in utils
- Make DCAE APP simulator store whole received messages history
- Add validation endpoint
- Add new messege type: FIXED_PAYLOAD

Closes ONAP-686

Change-Id: I865804716ad5e46a7503a8eee70cfe9ac75a6c1e
Signed-off-by: Jakub Dudycz <jakub.dudycz@nokia.com>
Issue-ID: DCAEGEN2-601
17 files changed