New group management module

There is a new group management module placed in the plugins folder:

  plugins/modules/ipagroup.py

The group module allows to add, remove, enable, disable, unlock und undelete
groups.

The group module is as compatible as possible to the Ansible upstream
`ipa_group` module, but addtionally offers to add users to a group and also
to remove users from a group.

Here is the documentation for the module:

  README-group.md

New example playbooks have been added:

  playbooks/user/add-groups-to-group.yml
  playbooks/user/add-user-to-group.yml
  playbooks/user/add-group.yml
  playbooks/user/delete-group.yml