Migrate "system login" to XML/Python representation
* [x] remove `system loginRemove the following CLI commands:
* Why should be add a user <user>to a 3rd party group` node, when the system is fully managed by CLI?
** Why should be add a user to a 3rd party group when the system is fully managed by CLI[x] remove `system login user <user> group` node
* There is only one privilege level, what is the sense in keeping this orphaned node?
** [x] remove `system login user <user> level admin`
** This is the only privilege level left and also the default, what is the sense in keeping this orphaned node?