IAMProvider accepts ECS IAM Task roles

[NickLavrov]

Per this issue #956

This PR adds the is_ecs_task arg to IAMProvider. When it is set to True, it follows a slightly different URL path to get credentials.

If you run a task on AWS ECS without a given task role, then curl http://169.254.170.2$AWS_CONTAINER_CREDENTIALS_RELATIVE_URI from within the container return a 404 error, and this would be caught by the error handling already in this class.

This would also require updates to the documentation on IAMProvider.

Since AWS_CONTAINER_CREDENTIALS_RELATIVE_URI is so long, the 80-char line limit was challenging :)

I have verified this works in an ECS task with an IAM role with:

client = Minio(
    "s3.amazonaws.com",
    secure=True,
    credentials=Credentials(
        provider=IAMProvider(is_ecs_task=True)
        )
    )

[balamurugana]

We can take this PR as is. I am working on adding WebIdentity provider and modifying IAM provider to support to complete.