Allow int parameters with the value "0" in ClusterController

Implementation of convertToIntOrUseDefault could never accept 0 as a valid value since it would evaluate
to false and return the default value.

The use case this fix addresses is I want to create the next ASG in the cluster at size 0, then scale it
up with more controls to account for autoscaling scenarios (Instance termination disabled, etc)
Merge pull request #560 from cfieber/master

Allow int parameters with the value "0" in ClusterController