Add support for EKS IAM ServiceAccounts

[lebenitza]

This is a feature request.

Currently wal-e works only by being provided with AWS Credentials, which being already deployed in AWS is a security nightmare. I am using wal-e with Zalando Postgres Operator and I tried to make it work with:

https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html

But without success. What do you think?