support IAM EC2 roles in S3 shared cache (from metadata service)
Summary
When configuring S3 shared cache, it fails with unauthenticated, even if the server has the correct EC2 IAM role
Steps to reproduce
Configure S3 shared cache, but do not set:
AccessKey = "MY_ACCESS_KEY"
SecretKey = "MY_SECRET_KEY"
Actual behavior
Shared cache doesn't work
Expected behavior
Shared cache works, by using the metadata service to create AccessKey and SecretKey