Add a configurable number of retries to the EC2 credential provider

In cases where the EC2 provider is configured as the only valid provider
of credentials, intermittent failures are to be expected and can be
retried safely in order to avoid a credential error.
Merge pull request #634 from menno/feature_retry-option-for-ec2-credential-provider

Add a configurable number of retries to the EC2 credential provider