ipaserver/vars/default.yml: Use python3-libselinux instead of libselinux-python

The python3 bindings should be required and not the python2 bindings as a
default.

roles/ipaserver/vars/default.yml

 # defaults file for ipaserver
 # vars/default.yml
-ipaserver_packages: [ "ipa-server", "libselinux-python" ]
+ipaserver_packages: [ "ipa-server", "python3-libselinux" ]
 ipaserver_packages_dns: [ "ipa-server-dns" ]
 ipaserver_packages_adtrust: [ "samba" ]