More Rubocop warnings fixed

It is now following `Fog` main repo style guide.


I changed this because of those rubocop warnings:

```ruby
lib/fog/core/attributes.rb:172:9: C: Prefer each over for.
        for arg in [:service] | args
        ^^^
lib/fog/core/attributes.rb:173:11: C: Favor modifier unless usage when having a single-line body. Another good alternative is the usage of control flow &&/||.
          unless send("#{arg}") || attributes.has_key?(arg)
          ^^^^^^
```

There are only changes necessary to satisfy rubocop. :smile: