Allow using IAM Profiles for etcd S3 credentials

[ethanchowell]

Describe the solution you'd like:
[A clear and concise description of what you want to happen.]
It would be great to remove the requirement for etcd s3 credentials for the scenario of users restricted to temporary credentials or usage of IAM profiles.

Why do you want this feature:
Currently if you don't provide credentials to the server config for S3 backups the controller will fail. For users that can only use temporary credentials this is problematic. A warning log saying something like "no credentials provided, relying on IAM" or even a flag to explicitly ignore providing credentials would be

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]