Use ResourceDefinition when updating Load Balancer

Use a ResourceDefinition object to define the Load Balancer configuration
when Autoscaling updates it. This makes it consistent with the regular
stack update code, which now passes a ResourceDefinition instead of a
template snippet.

Change-Id: I717559257b01bd0eaf0f3da48fb64bf60dcb2f8d
Merge "Use ResourceDefinition when updating Load Balancer"