Support step output variables in ARM templates

[mbpolan]

This is a take-away item from #3. A nice feature for the ARM runner would be to support a similar concept of propagated step variables like in the Terraform runner.

We can probably implement this by leveraging the template output feature via Azure CLI. The current implementation in #3 creates individual deployment metadata for each step, so we can most likely implement some logic to parse the outputs from previous steps and expose them as variables to later step ARM templates.