[#70437918] rubocop lint tool and associated fixes

This fixes all bar one of the errors highlighted by Rubocop's lint check.

The remaining issue:

````
lib/vcloud/core/vm.rb:115:65: W: Unused method argument - vars. If it's necessary, use _ or _vars as an argument name to indicate that it won't be used.
      def generate_preamble(script_path, script_post_processor, vars)
````

... is to be fixed in a subsequent PR, since the solution to is somewhat up for debate. 

As a result, this **does not** enable rubocop as a default target for Rake, since this would cause CI to fail.