Allow retrieval of passwords from KeyVault for Service Principal access

[peterhessey]

🚀 Feature

The setting up AzureML should provide an option to retrieve Service Principal passwords from KeyVault.

Motivation

Passwords should not be stored in source code.

Pitch

Add a flag to runner.py and any other required scripts that allows one to specify a keyvault URI to retrieve a service principal password from.

AB#6930

[ant0nsc]

I would suggest to switch the whole authentication story to use config.json like in hi-ml. With that, we would already get SP authentication out-of-the-box, with passwords stored in environment variables.