Don't pass around DO tokens as a variable

We're only using this for authentication in the provider, so follow best practices by relying on the environment variables to be properly set. Avoids persisting these tokens to the resulting state file.

Unfortunately, we still need the spaces access key / secret as variables because we pass those to K8s secrets. But relatedly:

1. why do we have the same secret data twice in the same namespace? They should share...
2. We should/could use separate access keys/secrets for authenticating Terraform & passing to k8s.