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
please take the distribution parts from the enviroment variables resp. set the target ones in the switch statement. try to prevent hardcoded paths like this.
Hint: there already is $slesversion, $slesdist, $slesmilestone. If that is not enough add another variable that can be used script wide, not only in your function.