In the OS pipeline, the parent configuration was used to detect if the
pipeline's setup was meant for an ostree commit or not. Also, the
pipeline used a new type to specify the ostree parameters.
- Use the ostree.CommitSpec for the parent configuration.
- Add a new attribute, OSTreeRef, that defines the ref for the ostree
commit being built. An empty string indicates that the tree is not
for an ostree commit.
Additionally, in the ImageKind configurations for the ostree archive and
container, separate the ostree ref from the parent spec, make the parent
spec optional (pointer) and the ostree ref mandatory, by requiring it in
the constructor of the ImageKind.
Implement all of Fedora in terms of this new abstraction. What used to be the
manifest functions (and before that the pipeline functions) are now the image
functions, whose purpose is to instantiate the right image kind structs from the
image type definitions we currently have in the distro definition.
