Feature/aws instance profile credentials

[Startouf]

The current version of Shoryuken later does not support instance profile credentials (from instance metadata service)

Actually, I am not sure how credentials are handled by other gems, and I don't think their presence should be checked at all...

Checking for instance profile credentials with Aws::InstanceProfileCredentials.new can be quite long (few secs) event with a single retry, I'm not really satisfied by this but it's better than nothing