Add Labels and Annotations to MetadataAccessor

Justification of this PR:
I need MetadataAccessor to be able to read & write Labels from runtime.Object. Then I could get rid of type-switches, as in https://github.com/openshift/origin/blob/master/pkg/config/config.go#L104.
I don't think this is necessary - real objects map have a nil map, and go makes it easy to deal with.