Support login with just the arm mixin

[carolynvs]

I want to be able to authenticate to Azure without using the az mixin. It should be a mixin configuration whether or not authentication occurs automatically.

mixins:
- arm:
    autoLogin: true # Automatically log into azure as needed

install:
- arm: 
      description: "This step will automatically authenticate for me"

Login with ambient environment variables such as AZURE_CLIENT_ID, etc

install:
- arm:
     description: "login"
     login

Explicit login

install:
- arm:
     description: "login"
     login:
       clientId: "something else"
       clientSecret: "than the default"
       tenantId: "azure environment variables"