Fix broken "large update" warning.

At some point, the "large update" warning logic was revised so that the
warning shows whenever you do an update that either:
- the updated config had more than 4x as many instances as the running one; or
- the updated config had less than 4x as many instances as the running one.

(Seriously: either local >= 4 * remote or local <= 4 * remote". And running git blame says it's all my fault.)

This fixes it: the correct logic is:
- the updated config has more than 4x as many as running; or
- the updated config has less than 1/4th as many as running.

Bugs closed: aurora-5860

Reviewed at https://reviews.apache.org/r/25450/