Define a convention for watching specific namespaces or all

[fabriziopandini]

User Story

As an operator, I would like to have a convention for deploying cluster API providers for watching objects in a specific namespace

Detailed Description

Most of the cluster API providers already support watching objects in a specific namespace, but there is no formal convention about how this should be exposed to the users.

By defining a convention, it will be possible to implement higher-level tools to automatically deploy any provider watching objects in a specific namespace

Goals

• To define a convention for requirements on how to expose to the users' options for configuring namespaced deployments

/kind feature

[fabriziopandini]

Similar to this:
Defining a convention for scaling up / down a provider, where scaling down to 0 == shut down the provider

[vincepri]

I stumbled upon this issue reading the clusterctl proposal, we've already started an effort to label external objects #1570

[vincepri]

/kind documentation

Let's document a convention that requires pluggable providers to provide flags to watch all or a single namespace.

[fabriziopandini]

WIP in #1994