Fixes #5190: Redirect the output of Rudder crontab to dev null

Ticket: http://www.rudder-project.org/redmine/issues/5190

To be merged in 2.10

Will require manual adjustment to 3.0
The bundle you've added this in is an old, deprecated bundle. See the comment above it:

```
 # This is to cleanup /etc/crontab from pre-2.5 usage
 # and initial promises before 2.7
```

We should therefore remove it in Rudder 3.0. And therefore, I don't think it's a good place to set the cleanup for AIX... Could you move it? You can probably just use a standard bundle edit_line delete_lines_matching(regex) (from the CFEngine stdlib).