TT#106950 check that sshd is listening on localhost

Some of our scripts and templates rely on sshd listening on the
localhost addresses (e.g. the port test in monitrc). Normally this is
achieved either by having an ssh_ role assigned to the `lo` interface,
or by listening the addresses explicitly in the `listen_addresses` list.
Add an explicit test for this in case it is missing.

Change-Id: Iee409551e8237fbdec56fe33d1bb63d56d0af802

templates/820_network-ports-debian.yaml.tt2

@@ -4,6 +4,10 @@ port:
     listening: true
     ip: ["0.0.0.0"]
 
+  tcp6:22:
+    listening: true
+    ip: ["::"]
+
   # Exim MTA
   tcp:25:
     listening: true