SystemDescription: Put global schema loading code into a method

This is slightly cleaner. We will also load scope schemas in this way in
the future.
SystemDescription: Build global schema path more cleanly
Add JSON schema validation for the "os" scope

Also add generic infrastructure for validating scope data (since "os" is
the first validated scope).
Merge pull request #126 from SUSE/json-schema-os

[Review] Request from 'dmajda' @ 'SUSE/machinery/json-schema-os'