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.