Feature/iotcore 110 restrict proxy function scope

Address the following issues:

https://github.com/telefonicaid/fiware-orion-pep/issues/7
https://github.com/telefonicaid/fiware-orion-pep/issues/6
https://github.com/telefonicaid/fiware-orion-pep/issues/3


Why you use 'x-forwarded-for' in the first case, then 'X-Forwarded-For' (different case) in the second? Just curiosity...