Support Delete operation for ds Passtrough-Running in DMI Plugin

See #3 in https://wiki.onap.org/display/DW/CPS-391Spike%3A+Define+and+Agree+NCMP+REST+Interface#CPS391Spike:DefineandAgreeNCMPRESTInterface-Datastore,PathsandFormatCombinationsforWriteOperations

and https://wiki.onap.org/display/DW/CPS-577%3A+Prepare+the+request+from+client+and+send+write+request+to+dmi-plugin

Implement Rest Endpoint POST method to request a "delete" operation towards DMI

requestBody will include the operation "delete" and other required details (identical to update method for same)

re-use generic methods in DMI Service for forward request and data to the SDN-C component. i.e org.onap.cps.ncmp.dmi.service.DmiServiceImpl#writeData

A/C

1. Target Resource is successfully deleted (can not be read again)
2. Agree and implement/document HTTP Error Code for invalid Operation 400/500?
   1. record on https://wiki.onap.org/display/DW/CPS+Exceptions+and+REST+API+HTTP+Response+Codes (add section for DMI Plugin)
3. Good en clear OpenAPI definitions include examples (not new, modifying existing ones!)
4. E2E test using CSIT (incl. demo to team, should be separate commit/subtask )
5. Update Release notes with OpenApi