Cloud4upgrade

Implementation of a upgrade subcommand to upgrade an existing cloud to the next version. Based on the work of @rhafer, with contributions of @freyes and @nkrinner
This target should rather be called "plain_with_upgrade" (it expands).
The prefixes "all", "plain", "instonly" are expanding to a set of steps. The name "upgrade" sounds like single step as well.

Maybe you could (additionally) introduce a step called upgrade an call the functions you are expanding to from there (additionally, so you can run it as one step or as several steps from the expanding "plain_with_upgrade" target).