We're currently using environment variables defined by the Azure auth library to authenticate (in Dial). However, there's another function provided that gets creds from the az CLI, which may also be useful. Let's add an environment variable that determines which to use.

Also, we should provide a default AZURE_AD_RESOURCE since the regular default is wrong (the Azure auth library defaults it to the "management" resource, which is different than the "keyvault" one).