Introduce the concept of addons

[grampelberg]

Feature Request

What problem are you trying to solve?

As linkerd adds new features (TLS, admission controller, #1890 #1887 #1307) and installs become more complex, the install command introduces more and more command line arguments that are a little difficult to keep track of.

It would be nice to have a set of "addons" that you can optionally install or remove from your installation. Many of the addons (maybe all of them today?) can be enabled by default. As an added benefit, it would be nice to install only these addons after the fact.

How should the problem be solved?

I'd like to see the modification of install and the addition of a CLI command.

For install, it could look something like:

linkerd install --addons=grafana,tls,ha

install would have a set of defaults out of the box (obviously, grafana would be a default). You could change up that list then if you'd like to.

Then, post-install, it would be possible to install these if you decide you'd like them later:

linkerd addon grafana

Open questions

• How would this interact with --single-namespace. Maybe --single-namespace actually becomes a reduction of RBAC addons?
• I'm not sure the post install use case is valid. You can just add/remove from the list of install and get the same benefit from Kubernetes.