Add DefaultAzureCredential as fallback in case of no auth config

[figglewatts]

This PR adds DefaultAzureCredential as a fallback in case no authentication method is configured in the INI file.

To implement this, I have:

• Removed the check for authentication methods in _validate_credentials(), as having no configuration is now valid.
• Changed the return value in the else case of _get_azure_credentials() from ManagedIdentityCredential to DefaultAzureCredential, as DefaultAzureCredential will attempt to use ManagedIdentityCredential anyway and this gets us a bunch of other authentication methods essentially for free (such as Azure CLI, shared token cache, Azure Powershell etc).
• Removed the test for missing authentication method config, as it's now valid to have missing authentication config.
• Silenced a couple of noisy loggers.

Looking forward to seeing what you think :)

[terricain]

Hey, this looks good, definitely should have used DefaultAzureCredential when they updated the DNS library.

The only issue I could see with this would be if lets say the VM had 2 user assigned identities, would it still work? Could you test that scenario. If its all good, happy to merge.

[terricain]

If you feel this is still relevant and can test that scenario I mentioned feel free to re-open this