Refactored 'user/*' roles into 'users' role

- this role is a very inefficient and needlesly repetitive group and
  user manager;

- 'users' role allows you to define different users and user groups in
  a set of hash variables with minimal syntax. Example can be found in
  'playbooks/roles/users/defaults/main.yml';

- this role looks stable, but there might be errors in "when:" logic in
  some tasks, further testing is required;