Rename 'spec' to 'config' (e.g. Generator.spec → Generator.config)

[wking]

While github.com/opencontainers/runtime-spec/specs-go is a reasonable name for the package, the Spec type there should really have been called Config (because it represents a particular config.json). It's probably too late to be worth changing in runtime-spec, but I think it is worth updating ocitools to use config in its internal names and public API while those are still fluxy. Thoughts?

[wking]

There is now a PR for this in #266.