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

[justinjstark]

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.

[justinjstark]

The current error I get on the next ApplyConfig is:

CloseError: Cannot apply configuration from slot 'Production' because this slot has already been prepared for swap with slot 'deploy'. 

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @antcp, @AzureAppServiceCLI

[panchagnula]

@Kotasudhakarreddy this is a good feature support to have tracking this for tentative October release if possible.