Limit scanning success events to be less spammy

[squaremo]

As reported in https://github.com/fluxcd/image-reflector-controller/pull/165#discussion_r694677790, sending an event on every successful scan is considered too spammy. Consider e.g., limiting those events to just the scans that discover new tags.

[marioapardo]

The other thing is that the event sends a text that there is no new image and thus be able to create an exclusion.