New netgroup management module

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

    plugins/modules/ipanetgroup.py

The netgroup module allows to ensure presence or absence of netgroup
and manage netgroup members.

Here is the documentation for the module:

    README-netgroup.md

New example playbooks have been added:

    playbooks/netgroup/netgroup-absent.yml
    playbooks/netgroup/netgroup-member-absent.yml
    playbooks/netgroup/netgroup-member-present.yml
    playbooks/netgroup/netgroup-present.yml

New tests for the module:

    tests/netgroup/test_netgroup.yml
    tests/netgroup/test_netgroup_client_context.yml
    tests/netgroup/test_netgroup_member.yml
    tests/netgroup/test_netgroup_member_absent.yml
    tests/netgroup/test_netgroup_member_case_insensitive.yml

Signed-off-by: Denis Karpelevich <dkarpele@redhat.com>