Post-deployment action: DNS name for App Gateway (and admin console)

[edburns]

Just as #170 allows you to do these DNS actions at initial deployment time, this issue enables the user to do the actions via CLI after initial deployment time. This follows our existing pattern of exposing features two ways:

1. initial deployment time via portal
2. after deployment time via CLI

[gnsuryan]

This work was put in the backlog, due to the issue faced during the configuration of Custom SSL setup for Configured & Dynamic Clusters with HTTP Listen port for Admin Server disabled and self-signed certificate configured. (Issue# 170)

Now that the issue is resolved, I will actively work on closing the issue#170

Once that is complete, which I can complete, hopefully by 1st week of Jan 2021, I will work on this issue#185.

I have already shared few inputs to Galia regarding DNS Zone/Alias configuration, which I had implemented already when enabling custom SSL feature for Admin Only offer.