Feature/conv op ngsi10 subscriptions

### Description
Implemented the ngsi10 convenience operation
POST /ngsi10/subscriptions

It simply uses the already implemented 'POST /ngsi10/subscribeContext'.
Complete with harness test (unit tests for subscribeContext were already implemented).

At the same time, a few "Overwriting entityId:id" for XML have been removed.
That part of the parsing doesn't exist any longer, so these pieces are old and not wanted.