naggen: avoid clash with saltmaster for packages

Alas, we had to remove ensure_resource because it does not really work
under puppet 2.7, but this caused a duplicate package declaration on
saltmasters.

Re-introducing an ugly define should help here.

Change-Id: I9d57baa46efcaecdd3bf3ead43d28e16de6b9f71
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>