cloud-launcher: use venv with openstacksdk installed

I noticed on the new bridge the cloud-launcher job is failing with

 openstacksdk is required for this module

which is because this is running on bridge with the system python, not
the ansible-venv python where we install Ansible and openstacksdk.
Set the interpreter so we use this instead.

Change-Id: Ie15680e0a1976567bebce6bcd9121b6221f54142

inventory/service/group_vars/cloud-launcher.yaml

@@ -1,2 +1,9 @@
 ansible_roles:
   - cloud-launcher
+
+# This is the Ansible / openstacksdk venv setup on the bastion host.
+# The roles from above are setup in /etc globally, but when running on
+# the remote host they require opentacksdk access.  This means it is
+# assumed that a cloud launcher is the pre-configured bastion host;
+# which is true.
+ansible_python_interpreter: '/usr/ansible-venv/bin/python3'