Additional validation on instance group configs in the role manifest

[mook-as]

This PR does a few things:

• Converts the app/validation tests to be driven by the YAML files, to make it easier to add more tests
• Add validation of instance-group-specific configuration templates (including variable use and sorting)
• Change the validation of global configuration templates to only allow things that are in actually used jobs (as opposed to any job in any referenced release)
• Move some tests from model/resolver to app/validation; this makes it possible to load not-quite-valid manifests. We should eventually move validation (but not normalization) out of the resolver.
• Make ErrorList actually implement error, so that we can print it out easier (without having convert).

Note that this will require changes to the UAA & SCF role manifests to conform to the new checks.

[cfdreddbot]

:white_check_mark: Hey mook-as! The commit authors and yourself have already signed the CLA.