New idoverrideuser management module.

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

    plugins/modules/ipaidoverrideuser.py

The idoverrideuser module allows to ensure presence and absence of
idoverrides for users and certificate members.

Here is the documentation for the module:

    README-idoverrideuser.md

New example playbooks have been added:

    playbooks/idoverrideuser/idoverrideuser-absent.yml
    playbooks/idoverrideuser/idoverrideuser-certificate-absent.yml
    playbooks/idoverrideuser/idoverrideuser-certificate-present.yml
    playbooks/idoverrideuser/idoverrideuser-present.yml

New tests for the module can be found at:

    tests/idoverrideuser/test_idoverrideuser.yml
    tests/idoverrideuser/test_idoverrideuser_client_context.yml