gssapi is only needed for OTP if keytab is used. The common case with
password does not require gssapi.

This change also fixes the new ansible 2.8 failure if gssapi is not
installed on the controller. Ansible 2.8 seems to also transfer and load
action plugins to the node if they are not used.

Fixed wrong use of ipareplica name in ipaclient inventory examples.
Added tier1 and tier2 replica handling.

The replica role is not able to work with version 4.5.

New information about requirements, limitations, installation and usage

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

The new section contains RHEL/CentOS 7.4+ and Fedora26+ for now.

This has been done to make clear that these are admin settings and to make
these settings consistent to ipaadmin_keytab.

The use od ipaclient_keytab for ipahost is not correct as the admin keytab
needs to be used here.