New inventory plugin

The inventory plugin compiles a dynamic inventory from IPA domain, filters
servers by role(s).

Usage:

Create yml file, for example `freeipa.yml`:

    ---
    plugin: freeipa
    server: server.ipa.local
    ipaadmin_password: SomeADMINpassword
    verify: ca.crt

Get compiled inventory:

    ansible-inventory -i freeipa.yml --graph