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.
@bkw: any reason, why we require a bump here? i am not sure if this breaks existing usage