Feature/iotcore 102 check action inside xml

Read the action type from the XML body of the request.
Ok, no problem. I didn't know that item was in your mind...

However, let's move to the collective mind of github issues :), as techdebt issue: https://github.com/telefonicaid/fiware-orion-pep/issues/3, for the future (i.e. not for this PR).