make uid_min and gid_min of login.defs configurable

This PR lets UID_MIN and GID_MIN in login.defs come from attributes/default.rb, defaulting to same values as before. In order to achieve that, I fixed some minor issues along the way:

- fix some rubocop issues
- update to chefspec 4.1.1
- bump sysctl dependency to 0.6.0 as described in the readme
- use quotes for file mode of login.defs template
- add a chefspec test for login_defs creation

Tell me if you want me to split these into smaller PRs.
Lazyness as a virtue - I just did not need that for my usecase ;-)

The lower limit of 1000 is much more often a problem, for instance if you create your admins at uid 2300+ with fixed uids. The next automatic install might add a user with uid 2305, and when you add the next admin user, it will conflict on that system. I can't envision a similar thing happening because of wrong max settings.
But feel free to add them, if you do ;-)