Don't fail if AWS credentials are not provided

Fog support IAM instance profiles, which provides credentials automatically to the host through the AWS metadata service which are rotated every few minutes. This removes the need to control access via managed credentials and instead rely on the properties of the environment, which improves the security as any individual credential that was obtained would be made inactive within the next few minutes. Can we remove the check for the credentials if they aren't provided and allow fog to use the environment?

No file or test is named in the issue. Start by tracing where Fog requires AWS credentials and how environment-provided credentials are handled; done means startup succeeds without explicit credentials while AWS instance-profile credentials remain usable, with coverage for both cases.