[#70437918] rubocop lint checking, and related fixes

This fixes all bar one of the errors highlighted by Rubocop's lint check.

In addition, whilst fixing the Lint errors, it was noted that we were using Rspec 'expect(thing) == value' incorrectly in some cases, missing the '.to' -- this was subsequently causing tests to pass incorrectly. These issues have been fixed up in this PR, along with a test logic issue discovered as a result.

The one remaining unfixed lint 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.