change logrotate policy, rotate every month, and keep logs forever

For performance analysis, it is mandatory to keep logs for a very long
time. Also it is sometimes required to analyse a bug that happened
more than one more ago, so we have no other choices than keep logs
and not remove them.

slapos/recipe/erp5/template/logrotate_entry.in

 %(file_list)s {
-  daily
+  monthly
   dateext
-  rotate 30
   compress
   notifempty
   sharedscripts