Add facility for AzureWebJobStorage

[d-s-dc]

Issue

As per the documentation, if we are using AzureWebJobsStorage environment setting, we can specify AzureWebJobsStorage__accountName instead of __blobServiceUri or __serviceUri.

The existing implementation doesn't have this facility and incorrectly throws an error incorrectly not recognizing the environment setting

Fix

Add the code to allow __accountName if the AzureWebJobsStorage environment setting is used.

[hallvictoria]

Thanks for raising this. We'll review the scenario and get a fix out.

[hallvictoria]

Closing in favor of #145