Inplatform 462 use ECS optimized AMI

I realized that we had hardcoded the (formerly) current AMI back when doing a fix with test, to make sure it didn't replace the ec2 agent at the time (to isolate the amount of changes we were making). We need to fix this, since this hardcoded AMI is different for different regions.

Instead of going back to using the ECS-optimized AMI (from the SSM parameter), I added the current AMIs we are using to variables. This helps us limit the amount of changes we make for a given deployment, since if the ami updated, we could be updating the EC2 instance even if we weren't planning to.

We'll have to figure out how to keep our instance updated, but for INPLATFORM-462, we should just make the changes we were planning to and then update the AMI at a later time.

I also added the tfvars files for test and prod, since I didn't see any secrets in them and I think it is more helpful than having the content in a readme.

@chelsea-gille - could you take a look?