Added feature to copy authorized_users from specified s3 bucket path.
Added feature to copy authorized_users from specified s3 bucket path.

Solved 2 problems:
1. Instead of using http(s), I can use the existing bucket to store authorized_users, e.g. use the secrets bucket
2. Merge with the local authorized_users instead of override, to allow login with the private key which used to create the EC2 instance.

holmesjr/elastic-ci-stack-for-aws,buildkite/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws