Adapt Hosted Che 'k8s-image-puller' to the upstream needs

[l0rd]

Is your enhancement related to a problem?

Image pulling is usually the workspace startup task that takes more time. And a typical Che workspace needs > 7 images to be pulled.

Describe the solution you'd like

We can pre-pull the default images used by Che workspaces on every node of the Kubernetes cluster where Che is going to run. This is something that we have implemented for the Hosted version of che (che.openshift.io) with the kubernetes-image-puller.

This issue is to adapt the k8s-img-puller to upstream che (if needed) and add the documentation to use in che-docs.

[l0rd]

Related to redhat-developer/rh-che#1428

[amisevsk]

This may require some work on the k8s-image-puller side as well, as the default case is designed to support impersonating multiple users through fabric8-oso-proxy. There is a 'local testing' option which is more in line with a common use case, but it should be improved to be more usable.

[ibuziuk]

The upstream repo under che-incubator - https://github.com/che-incubator/kubernetes-image-puller
docs - eclipse-che/che-docs#1085

[ibuziuk]

I believe we should close the issue once the doc PR is merged

[ibuziuk]

@rkratky @themr0c closing this issue - please review the docs PR eclipse-che/che-docs#1085