Entities deletion

At the end, the final solution to bug #225 ended in implementing a new feature to delete entities using updateContext DELETE with 0 attributes (or the corresponding DELET conv op in the entity URL).

The PR also include include some fixes in harness test:

* Using "ngsi9" and "ngsi10" in all convenience*.test filenames in order to classified its purpose
* Clean "NGSI9 polution" in test harness that are purely related with NGSI10 (this was a leftover of the old days in which both NGSI9 & 10 were coupled).