refresh ec2 credentials when it is expired
refresh ec2 credentials if it will be expired in 5 min
Do not cache credentials inside DefaultProvider as EC2Provider may
refresh the credentials before security token is expired.
Merge branch 'ec2_credential_provider_auto_refresh' of https://github.com/ThoughtWorksStudios/aws-sdk-ruby into ThoughtWorksStudios-ec2_credential_provider_auto_refresh