Fix AWS credentials handling

[ManuStoessel]

What went wrong:
When using AWS as the cloud provider for creating a cluster, then using the credentials from environment variables doesn't work. The options access_key_id and secret_access_key are required and Terraform complains, when they are not present in the cloud provider config. When specifying those options with empty strings or bogus values, then those values overwrite the values taken from the environment variables and cluster creation fails. Only providing the credentials directly via the Terraform config parameters access_key_id and secret_access_key worked (either directly as strings or passed via variables).

Expected behavior:
I would expect to be able to leave out the access_key_id and secret_access_key when using environment variables to provide the credentials. The behavior that the specified options within the Terraform config override the values from the environment variables was expected behavior and I just tried it out since Terraform complained when leaving out those items from the config.