Allow option to create a AzureClient from environment variables

[natemcintosh]

Goal

Add another option for creating the AzureClient, this time from environment variables.

Context

Currently, the only way to create an AzureClient is with a config file. However, in one case, I'm setting up a connection to Azure via environment variables, and I don't have a config file available to the running code. I would like to be able to create an AzureClient from the environment variables.

I imagine a copy of helpers.check_config_req() could be made, and then altered to instead check for the existence of environment variables.