bugfix: per-region configs didn't work with associate_public_ip

The construction like the following didn't work — you needed to specify subnet_id outside of region_config block
```ruby
    aws.region_config "us-west-2" do |region|
#### skipped ####
      region.subnet_id = "subnet-0b34307f"
      region.associate_public_ip = true
    end
```