Instead of constructing the list of credential providers manually, if we
use the default list we can take advantage of the AWS SDK checking the
environment and returning either the EC2RoleProvider or the generic HTTP
credentials provider, configured to use the ECS credentials endpoint.
Also, use the `defaults.Config()` function instead of `aws.NewConfig()`,
as this results in an initialised HTTP client which prevents a fatal
error when retrieving credentials from the ECS credentials endpoint.
Fixes#2960
Signed-off-by: Andrew Bulford <andrew.bulford@redmatter.com>
9690d843fa