add aws s3 sse config and aws_session_token support

[declan-fitzpatrick]

closes #198, #253, #254 and #261

• Please provide ability to use AWS_SESSION_TOKEN for S3 upload/download #198 - update logic for AWS credentials, and enable optional AWS_SESSION_TOKEN as a variable.
• Unable to build the docker image #253 - updates docker files with python venv
• [bug] Notification policy spelling #254 - fix notification typo
• AWS S3 upload with SSE #261 - allow optional AWS_S3_SSE to be passed in.
• updated docs