Clean up of 'users' role

- defaults/main.yml file was cleaned up, not used or outdated variables
  and comments were removed;

- you can now set 'users' variable to False to disable user management;

- set of users_* lists has changed. Current lists available to use in
  inventory:
  - 'users_root' - for root account (change shell and configure
    dotfiles)
  - 'users_default' - default user account created by Ansible (usually
    current administrator running the playbook)
  - 'users_admins' - special list for administrator accounts, can be
    used in inventory/group_vars/all.yml
  - 'users_groups' - special list for group creation, only used with
    tasks that create or remove groups (both normal and system groups)
  - 'users_list' - "global" list of user accounts, can be used in
    inventory/group_vars/all.yml
  - 'users_group_list' - list of users for a group of hosts (only one
    group is supported at a time)
  - 'users_host_list' - list of users on a particular host