config: DRY up labels with annotations reference
config: DRY up labels with annotations reference

from
https://github.com/opencontainers/image-spec/pull/635#issuecomment-291634824
it was clear that all the rules for `Labels` are the same as
annotations, so let's DRY that up a bit.

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

opencontainers/image-spec,vbatts/oci-image-spec,jonboulle/image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,jonboulle/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec