Update ami used

[glinton]

Currently, the ami's are hardcoded from December 2016. At least one uses a kernel with a known memory bug - it oom kills very aggressively.

We should either update the ami's used, or use the latest so we don't have to update the adapter every time AWS releases a new ami.

[tylerflint]

Do we know which amis we should be using? It's a pretty simple update.

[rgoomar]

If it has to be an Ubuntu image, then using the latest could work, but you can find the images here - https://cloud-images.ubuntu.com/locator/ec2/

But, the AWS CLI way to find the latest Ubuntu image is:

aws ec2 describe-images \
 --region us-east-1 \
 --filters Name=name,Values=ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-server* \
 --query 'Images[*].[ImageId,CreationDate]' \
 --output text \
 | sort -k2 -r \
 | head -n1

Something like that. Which you could just use that and iterate through regions to find the ami.
And if you want to pull out the ami, just add this after:

 | cut -f 1