Azure backend doesn't have file options for account_name and account_key

[neilcook]

The S3 backend has alternatives to access_key and secret_key being specified inline, by providing a way to specify a file containing these credentials. This is very useful for production deployments, where you don't want the credentials to be stored in a YAML config file.

The Azure backend should have equivalents, e.g. account_name_file and account_key_file.