Migrate "system login" to XML/Python representation
* [x] rRemove `system login user <user> group` node,
**the following CLI commands:
* 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 admin`, what is the sense in keeping this orphaned node?
** This is the only privilege level left and also the default, what is the sense in keeping this orphaned node?** [x] remove `system login user <user> level admin`