Add Kubernetes ExternalDNS to Addons

[burn2delete]

Is this a request for help?:

No, although direction on how to create an addon would allow me to get a PR started

Is this an ISSUE or FEATURE REQUEST? (choose one):

Feature Request

What version of acs-engine?:

master branch

Orchestrator and version (e.g. Kubernetes, DC/OS, Swarm)
kubernetes

What happened:
There is no asc-engine based deployment of external DNS services

What you expected to happen:
Deploy K8s ExternalDNS as an addon with the cluster

How to reproduce it (as minimally and precisely as possible):
n/a

Anything else we need to know:
n/a

[CecileRobertMichon]

You could look at #3198 for an example PR for adding an addon

[PatrickLang]

+1 :)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contribution. Note that acs-engine is deprecated--see https://github.com/Azure/aks-engine instead.