Add warning to periodic_task with interval 0

In Ia227f4c4e69ecf361ab02d1d17a3010303650104, warnings were
added for several options but instance_delete_interval was
left out by mistake.

Closes-Bug: #1188226
Change-Id: I20b396d664e40252743cac4f2a49f4f34882b32f
Merge "Add warning to periodic_task with interval 0"