Ubuntu support added (partly pick from pyguy)

This is a partly pick of the changes done by pyguy for pull request #28.
The unrelated changed on inventory files have been removed and the change
to the README has been adapted to use the "Supported Distributions" section.

The original commits by pyguy in https://github.com/pyguy/ansible-freeipa
are:

commit 1ed1fa845eafd69432b1fd1fc8e5329e4991e84a
Author: pyguy <hr.josheghani@gmail.com>
Date:   Mon Jun 25 17:17:31 2018 +0430

    Ubuntu Support added

commit 9a4a7c84e4af20af27e814aba4fc2c6b8b35ec0f
Author: pyguy <hr.josheghani@gmail.com>
Date:   Sun Jun 24 10:58:07 2018 +0430

    Ubuntu support added