Patch RS_DESTROY behavior

QA-723's implementation (puppetlabs/beaker#124) moved the logic for `@options[:preserve_hosts]` out of the `Beaker::NetworkManager#cleanup` method to `Beaker::CLI#execute!` and beaker-rspec doesn't use the CLI, so the logic would have to be duplicated here, or beaker would need a patch to move it back.

It doesn't really make sense for the cleanup method to conditionally cleanup, and it doesn't make sense for beaker to be inspecting the success state of RSpec, so beaker-rspec can be changed to work.