Add support for AWS SecretsManager to Docker images

Background

We now have an officially supported Docker image for Jira Software server (+ others) - good work Atlassian!

Request

When we deploy inside an AWS ECS environment (i.e. AWS' managed Docker service), we like to inject secrets (DB credentials, etc) via a service called AWS Secrets Manager.  This simply injects the decrypted secret value via an environment variable. Unfortunately it's a single, JSON-formatted string with multiple values

Suggestion

Add a variable called ATL_AWS_SECRETSMANAGER_DB_CREDS, and parse the individual values (host, port, user, pass, etc) from this, instead of passing individual values via individual env vars.