Add integer exit code for 'az deployment group what-if' command

**Is your feature request related to a problem? Please describe.**
When using the what-if feature in a deployment pipeline, it would be much easier to parse an integer exitcode that many lines of text. In a similar way to the Terraform command's [--detailed-exitcode](https://www.terraform.io/cli/commands/plan#detailed-exitcode) flag, which returns 0,1 or 2. This would allow a pipeline script to easily detect whether changes have occurred and make a decision based on the exitcode. The current Azure CLI requires iterating through results to check for drift. Checking the exit code is more standard and stable.

**Describe the solution you'd like**
A new parameter/flag to produce an integer from the what-if command to provide easy parsing to the consumer.

**Describe alternatives you've considered**
Parsing the 'what-if' textual output myself, which is error prone

**Additional context**
None