Allow overriding of all images used in Che deployment when using operator

Describe the bug

Today, the references to the plugin brokers (both of them? are both still needed/used?) are in

https://github.com/eclipse/che/blob/master/assembly/assembly-wsmaster-war/src/main/webapp/WEB-INF/classes/che/che.properties#L571-L574

Which means to update that to a new version, or to use a productized version, or a path-to-my-organization’s-airgapped-artifactory version, a user/customer has to hardcode new defaults and rebuild the Che server. This is awkward as it means literally forking the codebase just to point to a new container image.

Proposed solutuon

A better solution would be to have these values stored as env vars, as we do for other deployments like the devfile and plugin registries, so that they can be changed after deployment but before starting the first workspace.

And this decouples the versioning too, so that the same Che server can be used with a newer plugin broker, should a fix be needed in one but not the other.

We should also expose these env vars to the operator so that it can assign them via a custom resource.yaml file (as we do when deploying Che with chectl).

[nickboldt]

So we can set these env vars after deploying Che server?

• CHE_WORKSPACE_PLUGIN__BROKER_INIT_IMAGE
• CHE_WORKSPACE_PLUGIN__BROKER_UNIFIED_IMAGE

[amisevsk]

Updated title to reflect expanded scope (not just plugin brokers)