[users_and_groups] Refactor to fix and enhance users and groups management

users_and_groups role suffers of several coding weak designs or bugs (see here, here and here). It desserves a global refactor to sort out its present límitations.

Morover the rule "act once on host", I'll try to refactor the API in order to implement:

• groups definition with group and groups parameters as well as the present role's cielito_groups management,
• ability to set certain user parameters at a group level, defining them for all group members,
• a structure of default values for users and groups.