create a command to generate a list of all images used

[jstrachan]

similar to #278 it would be nice to create a step to process all the sources in config-root to find all the image tags in every Deployment / Pod etc and create a YAML file with all the images (and which resources they are referenced from)

this would help folks grok what images are needed; what registries are used, what versions are used etc

folks could use this data to then make further tools such as:

• ensuring all images are accessible
• mirroring / replicating images to other registries to support air gapped on premise type repositories or to work around locations which cannot access specific registries. e.g. in China its often hard to access gcr.io

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