Debian Buster: Fix "No module named 'ipapython'".

When using ipaclient role under Debian 10 (Buster), it is required
that the python interpreter is set to Python 2.7 as freeipa-client
package pulls in Python 2.7 dependencies, and does not work with
Python 3.

This patch adds configuration to properly set python interpreter.

Based on the work by Marc Richter (@The-Judge on Github).

Fix issue #607.

roles/ipaclient/vars/Debian-10.yml

+---
+# vars/Debian.yml
+ipaclient_packages: [ "freeipa-client" ]
+# Debian Buster must use python2 as Python interpreter due
+# to the way freeipa-client package is defined.
+# You must install package python2.7 before executing this role.
+ansible_python_interpreter: '/usr/bin/python2'