New idoverridegroup management module.

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

    plugins/modules/ipaidoverridegroup.py

The idoverridegroup module allows to ensure presence and absence of
idoverrides for groups.

Here is the documentation for the module:

    README-idoverridegroup.md

New example playbooks have been added:

    playbooks/idoverridegroup/idoverridegroup-absent.yml
    playbooks/idoverridegroup/idoverridegroup-present.yml

New tests for the module can be found at:

    tests/idoverridegroup/test_idoverridegroup.yml
    tests/idoverridegroup/test_idoverridegroup_client_context.yml