Fixes #7450 - do not mark non-required fields

If validation is conditional we don't mark fields as required by
default. Also fixes disabling by override using :required option.
@ohadlevy, I'm ok with merging @ares's PR. I can see the point of preferring to **not** show * and the form will give a validation error that it's required, than the opposite to make users think a field is required when it's not.