Requirements like being able to run minio gateway in ec2
pointing to a Minio deployment wouldn't work properly
because IAM creds take precendence on ec2.
Add checks such that we only enable AWS specific features
if our backend URL points to actual AWS S3 not S3 compatible
endpoints.
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.
