Use DefaultAzureCredentials for workspace auth

Apparently, it is not possible to use DefaultAzureCredentials for workspace auth.

From the [docs](https://github.com/Azure/MachineLearningNotebooks/blob/master/how-to-use-azureml/manage-azureml-service/authentication-in-azureml/authentication-in-azureml.ipynb), the available auth types are:
Interactive Login Authentication
Azure CLI Authentication
Managed Service Identity (MSI) Authentication
Service Principal Authentication
Token Authentication

It would be very useful if this SDK used the standard Azure way to handle authentication