New user management module

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

  plugins/modules/ipauser.py

The user module allows to add, remove, enable, disable, unlock und undelete
users.

The user module is as compatible as possible to the Ansible upstream
`ipa_user` module, but addtionally offers to preserve delete, enable,
disable, unlock and undelete users.

Here is the documentation for the module:

  README-user.md

New example playbooks have been added:

  playbooks/user/add-user.yml
  playbooks/user/delete-user.yml
  playbooks/user/enable-user.yml
  playbooks/user/disable-user.yml
  playbooks/user/delete-preserve--user.yml
  playbooks/user/undelete-user.yml