Allow 'load_balancers.*' to pass resource_aws_autoscaling_group_validation

Fixes #206

When the load_balancers arguments is used in an aws_autoscaling_group instance, one gets:
```
There are warnings and/or errors related to your configuration. Please
fix these before continuing.

Errors:

  * 'aws_autoscaling_group.ingress_nodes' error: Unknown configuration: load_balancers.#
  * 'aws_autoscaling_group.ingress_nodes' error: Unknown configuration: load_balancers.0
```