IAM support

Anticipates support for this in official upstream. This should be a no-op for the previous version given the new `node[:wal_e][:use_iam]` is set to false.

There is a fork which supports IAM : https://github.com/drax68/wal-e/commit/979072b7d8265ebac4d04d3b9e5d92d361b56a5d