webapp: Cancel a "Swap with Preview" deployment swap after ApplyConfig

## Description of the new feature
From #3667. I'd like to be able to cancel a "Swap with Preview" deployment swap after the -ApplyConfig step. This does not appear to be supported with Azure Powershell.

My deployment process is:
1) Create a new deployment slot
2) Deploy to deployment slot
3) Apply production config to deployment slot (swap with preview)
4) Wait for deployment slot to become healthy **
5) Complete deployment slot swap

If step 4 fails, my automated process is stuck. I have to log into Azure Portal and manually Complete Swap -> Cancel Swap.

** I use this method because app startup often takes 4 minutes for my app which prevents me from using the non-preview swap which has (I think) a 90 second timeout.