Task/581 v1 context entities attributes 4ops

### Description
Four new conv ops:
GET/PUT/POST/DELETE /v1/contextEntities/type/TYPE/id/ID/attributes/ATTR_NAME

Some tunning on the test in commits https://github.com/telefonicaid/fiware-orion/commit/46b544fd19d0d447505310dfc8513f9560726ecc and https://github.com/telefonicaid/fiware-orion/commit/6831800a019d4e33398d8de20e83ccb4f5f06410