Use a single DNS provider for external-dns for consistent naming across applications

[nacx]

With the current external-dns approach, we are creating one subzone per cluster. This leads to non-ideal DNS configurations when you have a demo environment with clusters in multiple clouds, as hostnames will have different DNS suffixes depending on where services are deployed. In the first version, I proposed (still working in my fork), external-dns was configured to control a single DNS zone (the one used by the MP), and all services in the environment got consistent naming.

Now, in a setup with, say, 4 clusters, 2 in AWS, 2 in GCP, I'm getting DNS names like:

tsb:  nacx161.mp.aws.tetrate.com
app1: app1.eks-nacx161rc2-eu-west-3-0.mp.aws.tetrate.com
app2: app.gke-nacx161rc2-europe-southwest1-0.gcp.sandbox.tetrate.io

We should have a way to configure external-dns to just use one zone so we can have consistent DNS naming in the deployed apps.