Enable authentication via environment variables

**Is your feature request related to a problem? Please describe.**
I'm concerned that I cannot use the CLI without having my credentials written to disk in the clear.

**Describe the solution you'd like**
I would like the CLI to accept credentials from the environment (e.g. `AZURE_TENANT_ID`, `AZURE_CLIENT_ID`, `AZURE_CLIENT_SECRET`).

**Describe alternatives you've considered**
I've yet to find a way to authenticate that doesn't involve secrets being written to `~/.azure`.

**Additional context**
I came across [this](https://github.com/Azure/azure-cli/issues/9282) issue which seems to describe the same thing, but was closed following an apparent misunderstanding. While the AWS CLI does indeed write secrets in the clear to disk when configured via `aws configure`, it also allows for [configuration via environment variables](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html).

I've also found examples in both the Ruby SDK [here](https://github.com/Azure/azure-sdk-for-ruby#option-1---environment-variables), and the Go SDK [here](https://github.com/Azure/azure-sdk-for-go#more-authentication-details) where authentication via environment variables is enabled.

This issue is particularly problematic in a dockerized environment where it's common for the home directory to be mapped on the host.