Image cmd enhancements for new worker nodes added by node cmd

[krotz-dieter]

The issue #39 introduces the possibility to add and remove new nodes. Within this issue the image cmd needs to be enhanced so that it supports these additional nodes.

In case of the image cmd the following subcommand need to consider also the additional worker nodes:

• ls : show also container images for all additional worker nodes
• export: export also form the worker nodes
• import: import also to the additional worker nodes
• pull: being able to pull also from additional worker nodes
• clean: being able to clean images also from additional worker nodes
• registry: save credentials and registry info on additional worker nodes
• rm: being able to remove images also from additional worker nodes