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