Use custom AMI when launching EMR clusters

[mreid-moz]

EMR 5.7 introduced the possibility of using a custom AMI for launching EMR clusters.

It would be great to take advantage of this to both use up-to-date OS images (currently 5.8.0 uses a fairly stale 2017-03 amazon linux image) as well as potentially speeding up cluster provisioning by baking some or all of the bootstrapping code into the AMI itself.

[mreid-moz]

cc @jasonthomas per our discussion earlier.

[mreid-moz]

This could also be used to roll out new images to stage, test them, then deploy the image to prod (rather than messing with the S3-based bootstrap scripts).