Parameterize CodeBuild images and bump default image
Added support to pass in different images for build and e2e stages. Bumped default build_image from aws/codebuild/standard:2.0
to aws/codebuild/standard:3.0 to get on a maintained and hopefully cached image as we saw pipelines relying on aws/codebuild/standard:2.0 slow down. https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-available.html