Allow different scc: subsections for different architectures or credentials

[Bischoff]

Currently, the scc: section contains only one list of architectures that are applied to all products. It makes it impossible to get from a single yaml file different architectures for different products. That is possible from the http: section.

Also, there's only one pair of SCC credentials for the whole section.

Align with http: by allowing subsections in scc: block.