bind: Handle conffile prompt during upgrade

- Simply keep the old configuration as the configuration format for bind is
  unlikely to change due to its stability. Future versions may consider checking
  for version of package being upgraded to.

- Don't start service if not already running.

- Don't perform firewall changes.

- No need to rerun setup as old configuration is being kept.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/bind/__init__.py

@@ -104,6 +104,11 @@ def setup(helper, old_version=None):
     helper.call('post', actions.superuser_run, 'bind', ['setup'])
 
 
+def force_upgrade(helper):
+    """Force upgrade the managed packages to resolve conffile prompt."""
+    helper.install(managed_packages, force_configuration='old')
+
+
 def enable():
     """Enable the module."""
     actions.superuser_run('service', ['enable', managed_services[0]])