Provide zero-configuration domain name for Auto DevOps using nip.io

Description

When you set up Auto DevOps, you need a wildcard domain name to reach your deployments. This domain needs to point to the Ingress IP address for your cluster.

There are public free services, like nip.io or xip.io that provides exactly what Auto DevOps need, and they are zero-configuration. You just need the ip address, and you can then build any kind of hosts:

*.192.0.2.23.nip.io -> 192.168.0.2

Proposal

1. When user has not set up a discrete domain, automatically set the AUTO_DEVOPS_DOMAIN value to the <ip>.nip.io domain as soon as an ingress is added to the cluster, or if a cluster is added and an ingress is detected.

2. In case of multiple clusters, we can set the domain with the value of the * cluster, and set environment-specific variables for the others.

3. Advise users (i.e. help text) is not mandatory to enter domain, we can do it for them automatically after they add the cluster (with the caveat that their ingress address must be in IP form, inform if otherwise)

4. for EE, this has to be done for each cluster that is added.