v1.16.0

fix: reserved instance multi_az setting @oycyc (#248)

## what

Using local.is_regional_cluster to determine whether or not it is Multi-AZ is not accurate. local.is_regional_cluster only checks if var.cluster_type == "regional", which determines if this is a regional vs global cluster

I could have a regional cluster, but it is NOT multi-az. One determining factor is how many instances in the cluster, if ever more than 1 in a single cluster, then it has to be in multiple AZs. https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.MultiAZ.html

why

If I have a regional cluster, this thinks that it's ALWAYS multi-az, which is not true.

references

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.MultiAZ.html