Utilize TF `locals` feature in order to implement instance switch option

What

• Utilize TF locals feature in order to implement instance switch option

Why

• We shouldn't use boolean value with count attribute. TF interpolates "true"/"false" strings as booleans, but can't interpolate string -> boolean -> int
• When we use terraform-aws-ec2-instance in order to create replicas, we might be interested in switch for those replicas, or replica groups