Support Azure Managed Identities for Blob Storage

In AWS, we have the concept of IAM instance profiles (use_iam_profile) so that no static credentials are needed to access object storage.

We have customers that will likely want the equivalent for Azure: https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/overview. We should add support for this.

To get this working, we probably need a similar configuration setting, such as use_managed_identity to tell Rails and Workhorse to access Azure even without static credentials.

References:

1. https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/tutorial-linux-vm-access-storage-access-key
2. Ruby SDK: https://github.com/Azure-Samples/resources-ruby-manage-resources-with-msi/
3. Golang: https://github.com/Azure/azure-sdk-for-go#more-authentication-details