Support ClusterImageMaps

[glyn]

Sometimes it is necessary to provide a relocation mapping that will apply to all namespaces. So the namespaced ImageMap resource should be augmented with a cluster-wide ClusterImageMap.

When an ImageMap is inconsistent with a ClusterImageMap, this is not an error and the ImageMap should take precedence. This allows specific namespaces to override the cluster-wide behaviour when necessary, e.g. for testing a later version of the images in the ClusterImageMap.