Make sure /usr/bin/python is present for mailman

Mailman v2.1 is still a Python2-only application, and expects
/usr/bin/python to be present. On Ubuntu Focal, there is no such
symlink provided by the Python 2.7 packages, and an extra
python-is-python2 transitional package is used to explicitly create
it in cases where that's required.

Change-Id: I37ca2bd7011afdb3b97e34cdc24ff455b9fb0498

playbooks/roles/mailman/tasks/main.yaml

@@ -21,6 +21,7 @@
   package:
     name:
       - mailman
+      - python-is-python2
     state: present
 
 # Install index.html, robots.txt