ipa[server,replica,client]: Add support for CentOS-8

The files for RHEL-8 (RedHat-8.yml) have simply been linked to CentOS-8.yml
for the ipaserver, ipareplica and ipaclient roles.

Fixes issue #121 (roles/*/vars needs CentOS-8.yml files)

roles/ipaclient/vars/CentOS-8.yml

+RedHat-8.yml
\ No newline at end of file

roles/ipareplica/vars/CentOS-8.yml

+RedHat-8.yml
\ No newline at end of file

roles/ipaserver/vars/CentOS-8.yml

+RedHat-8.yml
\ No newline at end of file