Improve ECS task deployment to spread across 2 availability zones

[hellais]

We should fix the task placement and deployment so each service in ECS has two tasks associated to them and each is in a different availability zone.

This should also address the problem identified as part of #250

[DecFox]

We already have some code which controls the task placement for ecs services:
https://github.com/ooni/devops/blob/main/tf/modules/ooniapi_service/main.tf#L106-L109

Since we use the bridge network mode, the target groups are mapped to ec2 instances which means that the location of the instance also decides where the task spins up. The ASG takes care of this internally. The only requirement for this is to pass in subnets for the AZs we want which we already do:
https://github.com/ooni/devops/blob/main/tf/modules/ecs_cluster/main.tf#L197