Code cleanup.

Remove useless statements and improve debug message.
Find out user groups and sudo status during login.

When the /login REST API is called with a valid username and password,
Kimchi will find out the user groups and sudo status, store these values
in sessions variables, and return them in a JSON message with the
following format:

{"sudo": true | false, "userid": "<username>", "groups": [<list of
groups>]}

For example:

{"sudo": false, "userid": "guest", "groups": ["guest", "foo"]}

This JSON return message can be used by the UI to figure out which
tabs/options should be shown in the web interface according to the user
rights.

It is important to notice that sudo configuration for a given user is
not as simple as an on/off key. Sudo permits various levels of access
controls. For instance, it is possible to specify which executables a
user can run with sudo. For the purpose of this implementation, we only
consider a user to have sudo rights if this user can run any command in
the system with sudo (which is equivalent to have root rights).
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 user access to REST API /host.

There is a need to restrict access to the REST API /host on POST, PUT,
and DELETE HTTP methods to user with admin rights. In the context of the
authorization feature developed, that means the user needs to have sudo
rights to run any commands on the system in order to be able to access
the /host REST API using POST, PUT, and DELETE HTTP methods.