ns-migration: fix courtesy policies

Avoid errors like:

  Traceback (most recent call last):
    File "/usr/share/ns-migration/20wan", line 87, in <module>
      mwan.store_policy(u, f'{i.get("name")}_only', [i])
    File "/usr/lib/python3.11/site-packages/nethsec/mwan/__init__.py", line 251, in store_policy

packages/ns-migration/files/scripts/wan

@@ -84,7 +84,9 @@ mwan.store_policy(u, policy_name, interfaces)
 
 # generate one policy for each interface to ease divert rules creation
 for i in interfaces:
-    mwan.store_policy(u, f'{i.get("name")}_only', [i])
+    pname = i.get("name")
+    # make sure to not exceed 12 chars
+    mwan.store_policy(u, f'{pname[0:6]}_only', [i])
 
 # divert rule are not migrated