Whitelist ansible_python_interpreter for add_host

It is possible for a user wanting to change the
ansible_python_interpreter when using add_host, this allows for that.

Change-Id: If07f66cc6a296aea817a06b229f933fec76c0adc
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

releasenotes/notes/add-host-ansible-python-interpreter-9592aef8d2c90b3e.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    The ansible_python_interpreter variable is now whitelisted for the
+    add_host task.

zuul/ansible/base/action/add_host.py

@@ -22,6 +22,7 @@ class ActionModule(add_host.ActionModule):
     def run(self, tmp=None, task_vars=None):
         safe_args = set((
             'ansible_connection',
+            'ansible_python_interpreter',
             'ansible_host',
             'ansible_port',
             'ansible_user',