[aws|elb] Mimic real behavior in create_load_balancer mock

This PR mimics the behavior like the real one, as a availability_zones can be nil on real class and not in mock class.

At this moment, I ran into an error in `availability_zones.first.gsub(/[a-z]$/, '')` in a Chef elb cookbook that assumes availability_zones as an optional argument, so its default value it's `nil`.

I tried to write a spec to catch this behaviour, but it was difficult to me.