fix defaults not handled by yaml configuration

[crazy-max]

related to #887

Current implementation of defaults is not working because the model.Image struct does not support proper yaml and json tags for the configuration.

Need a dedicated struct for this and also remove some fields that I don't think are necessary. Also move imageDefaults from watch to defaults in config root.

[IamTheFij]

I like the idea of these being in their own section! I'm not sure I understand why it needs it's own struct though. It looks like the model.Image struct has field tags for yaml and json already. Using a new struct means a bit more overhead if new fields are added (probably unlikely, but still).

[crazy-max]

why it needs it's own struct though. It looks like the model.Image struct has field tags for yaml and json already. Using a new struct means a bit more overhead if new fields are added (probably unlikely, but still).

That's because the configuration loader does not support tags with special chars like _ that cannot be translated automatically to env vars like max_tags must be maxTags so env var would be DIUN_DEFAULTS_MAXTAGS.