allow receive command notifications from CB

[AlvaroVega]

This PR does not modify current commands based on CB registrations. It's backward compatible and adds a new way of receive command through a CB notification.
related and needs telefonicaid/iotagent-node-lib#1560

• Doc already in related PR allow receive commands by CB notifications iotagent-node-lib#1560
• Test

[AlvaroVega]

Example of NGSIv2 Notification command:

curl -i -X POST http://localhost:4052/notify -d '{
    "subscriptionId": "60b0cedd497e8b681d40b58e",
    "data": [{
   	 "id": "123456abcdefg",
   	 "type": "switchOnOffExecution",
   	 "targetEntityId": {
   		 "type": "Text",
   		 "value": "ENT-LUM-001",
   		 "metadata": {}
   	 },
   	 "targetEntityType": {
   		 "type": "Text",
   		 "value": "StreetLight",
   		 "metadata": {}
   	 },
   	 "execTs": {
   		 "type": "DateTime",
   		 "value": "2020-05-27T00:00:00.000Z",
   		 "metadata": {}
   	 },
   	 "cmd": {
   		 "type": "Text",
   		 "value": "switchOnOff",
   		 "metadata": {}
   	 },
   	 "params": {
   		 "type": "Text",
   		 "value": "on",
   		 "metadata": {}
   	 },
   	 "status": {
   		 "type": "Text",
   		 "value": "FORWARDED",
   		 "metadata": {}
   	 },
   	 "info": {
   		 "type": "Text",
   		 "value": null,
   		 "metadata": {}
   	 },
   	 "onDelivered": {
   		 "type": "Request"
     	 },
   	 "onOk": {
   		 "type": "Request"
   	 },
   	 "onError": {
   		 "type": "Request"
   	 },
   	 "onInfo": {
   		 "type": "Request"
   	 },
   	 "cmdExecution": {
   		 "type": "value",
   		 "value": true,
   		 "metadata": {}
   	 },
   	 "dateExpiration": {
   		 "type": "DateTime",
   		 "value": "2020-05-27T20:00:00.000Z",
   		 "metadata": {}
   	 }
    }]
}' -H 'content-type: application/json' -H 'fiware-service: smartcity' -H 'fiware-servicepath: /'

[fgalan]

LGTM