Allow ability to configure whether built Docker image should be kept

[h-arlt]

Once the built Docker image has been successfully pushed to the configured registry, the question arises whether it should be kept in local registry at all.

As this might differ from situation to situation, the user should be able to configure the desired behaviour. Thus, we should introduce a new user variable, say keep_image, that defaults to true and pass its value to Packer's docker-push post-processor.

[h-arlt]

There is a bug - marked as 'wont-fix' - which prevents us from extracting the keep_input_artifact parameter of docker-push post-processor as user variable (it only accepts bool values but user variables are string -> string mappings).

Only solution would be transform all packer scripts and variable files to the new HCL2 format (still in beta).