Add image sync fields to schema

[enolfc]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [ ] If applicable, I’ve added docs and tests

Description of changes

Added new fields for image synchronisation (see https://github.com/EGI-Federation/fedcloud-catchall-operations/pull/350)

[enolfc]

Not the list of images, but whether to configure image synchronisation or not in the catchall instance.

[enolfc]

See sample config https://github.com/EGI-Federation/fedcloud-catchall-operations/blob/5f01874961fb46a5f98e4cb4946fa3422718f560/sites/WALTON-CLOUD.yaml