Feature: Ability to share waggledance endpoint to other VPCs via VPC Endpoint

[mroark1m]

I have a use case where it would be convenient for me to share a single waggledance service across two VPCs with a VPC endpoint. This is difficult to do manually, outside apiary-federation config, because the IP addresses of the waggledance service change on every update and there is no ELB. This issue is to investigate on how to enable this. Current thought, after discussion with @rpoluri, is to set up an optional ELB for waggledance within the apiary-federation config so that we can take advantage of auomated ECS -> ELB registration , but keep the VPC service set up outside apiary-federation.

[mroark1m]

Above design (optional ELB) does not work because the load balancer is specified as an inline block in the ecs_service. Until we get to Terraform 12.0 (and dynamic blocks https://www.hashicorp.com/blog/hashicorp-terraform-0-12-preview-for-and-for-each), there is no supported way to do this. There is some workaround involving a map, but from what I've read it doesn't work for this specific case, load_balancer block in an ecs_service (ref: https://www.reddit.com/r/Terraform/comments/8uoavs/conditionally_set_load_balancer_field_on_aws_ecs/)