authoritative-source shouldn't be required for images located on docker hub

[hhorak]

I'm thinking what authoritative-source label should be for images that are located on Docker Hub at https://github.com/projectatomic/ContainerApplicationGenericLabels/blob/master/vendor/redhat/labels.md.

If authoritative-source is only meant to be able to reconstruct the docker pull ... command, if authoritative-source is empty, then it results in correct docker pull ... command in case of docker hub hosted images. So, for those images the label authoritative-source shouldn't be required.

[rhatdan]

@riekrh @aweiteka PTAL