installation: cerberus package upgrade

* Upgrades Cerberus package in order to support some new features.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
restful: fix for testing accesstoken

* Renames class RestValidator to RESTValidator.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>