Task/include entity id and type as measures when ngsi payload

[AlvaroVega]

Continuation of PR #804

Given a ngsiv2 measure like:

curl -i -X POST 'http://localhost:7897/iot/json?i=test1&k=bryippqcmkxc8ncguqxejibts' -d'{
                "actionType": "append",
                "entities": [
                    {
                        "id": "urn:ngsiv2:Streetlight:Streetlight-Mylightpoint-2",
                        "type": "Streetlight",
                        "name": {
                            "type": "Text",
                            "value": "MyLightPoint-test1"
                        }
                    }
                ]
            }' -H 'content-type: application/json'

id and type from entity ( "id": "urn:ngsiv2:Streetlight:Streetlight-Mylightpoint-2" and "type": "Streetlight",) are progressing as measures to iotagent using prefix ngsiv2_id and ngsiv2_type respectively:

    "measure_id": {
        "type": "string",
        "value": "urn:ngsiv2:Streetlight:Streetlight-Mylightpoint-2"
    },
    "measure_type": {
        "type": "string",
        "value": "Streetlight"
    },

[mrutid]

LGTM

[fgalan]

The NGSI constant should be removed from constant.js file, shouldn't it?