Enhance UrlSubNode decorator and kimchiauth tool to check for sudo rights.

kimchiauth tool used to only check if the user was authenticated or not.
Now it also checks whether the REST API being accessed is only allowed
to users with sudo rights.

The necessity to have sudo rights to access a REST API can be easily
configured through the UrlSubNode decorator. Similar to the support
previously implemented for user authentication in UrlSubNode, an
additional boolean parameter was added to UrlSubNode to indicate whether
the user needs sudo rights in order to access the corresponding REST
API.
Limit REST API /host to user with sudo rights.