Add an option to use the existing EC2 instance for the Redis host, rather than creating an ElastiCache instance

[Kapps]

Roughly half the cost of funcie right now is the ElastiCache instance. If we just use an ECS task, we can cut the cost of funcie in half. It's not as good as using the separate instance, but for development loads it should be okay and users could choose larger or separate instances if they wish.