Parameterizing more values so it's more versatile

[visokoo]

Contains breaking changes.

Parameterizing more of the S3 bucket path so users can be more granular when automating with this container.

• Added S3_BUCKET, SRC_S3, S3_FILE.
  -- S3_BUCKET = Bucket name without slash
  -- SRC_S3 = Folder path in bucket
  -- S3_FILE = (Only used with sync-s3-to-local) A mash of both S3_BUCKET and SRC_S3 to find the latest S3 file in the bucket.

• Changed env vars to be in caps for best practice so this is a breaking change.
  -- aws_key => AWS_ACCESS_KEY
  -- aws_secret => AWS_SECRET_KEY
  -- cmd => S3_CMD

• Added if logic in sync-s3-to-local command so that it'll check for whether or not the backup exists before attempting to download again. If it does exist, echo that and exit 0.

[mvasin]

@visokoo Interestingly, POSIX standard doesn't claim that all-caps for environment variables of regular programs is the best practice:

Environment variable names used by the utilities in the Shell and Utilities volume of IEEE Std 1003.1-2001 consist solely of uppercase letters, digits, and the '_' (underscore) ... environment variable names containing lowercase letters is reserved for applications.

But it's widely used, for sure.