Allow minio s3 gateway to use aws environment credentials,
IAM instance credentials, or AWS file credentials.
If AWS_ACCESS_KEY_ID, AWS_SECRET_ACCSES_KEY are set,
or minio is running on an ec2 instance with IAM instance credentials,
or there is a file $HOME/.aws/credentials, minio running as an S3
gateway will authenticate with AWS S3 using those one of credentials.
The lookup order:
1. AWS environment varaibles
2. IAM instance credentials
3. $HOME/.aws/credentials
4. minio environment variables
To authenticate with the minio gateway, you will always use the
minio environment variables MINIO_ACCESS_KEY MINIO_SECRET_KEY.
To pass a custom S3 endpoint in S3 gateway, the user needs
to specify it as an argument after 'minio gateway s3' and not
as '--address' option since this latter specifies the address
to which the gateway should listen.
