➕ Add ConfigMap exercise

slides/exercises/appconfig-brief.md

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+## Exercise — Application Configuration
+
+- Configure an application with a ConfigMap
+
+- Generate configuration file from the downward API

slides/exercises/appconfig-details.md

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+# Exercise — Application Configuration
+
+- We want to configure an application with a ConfigMap
+
+- We will use the "rainbow" example shown previously
+
+  (HAProxy load balancing traffic to services in multiple namespaces)
+
+- We won't provide the HAProxy configuration file
+
+- Instead, we will provide a list of namespaces
+
+  (e.g. as a space-delimited list in a ConfigMap)
+
+- Our Pod should generate the HAProxy configuration using the ConfigMap
+
+---
+
+## Setup
+
+- Let's say that we have the "rainbow" app deployed:
+  ```bash
+  kubectl apply -f ~/container.training/k8s/rainbow.yaml
+  ```
+
+- And a ConfigMap like the following one:
+  ```bash
+  kubectl create configmap rainbow --from-literal=namespaces="blue green"
+  ```
+
+---
+
+## Goal 1
+
+- We want a Deployment and a Service called `rainbow`
+
+- The `rainbow` Service should load balance across Namespaces `blue` and `green`
+
+  (i.e. to the Services called `color` in both these Namespaces)
+
+- We want to be able to update the configuration:
+
+  - update the ConfigMap to put `blue green red`
+
+  - what should we do so that HAproxy picks up the change?
+
+---
+
+## Goal 2
+
+- Check what happens if we specify a backend that doesn't exist
+
+  (e.g. add `purple` to the list of namespaces)
+
+- If we specify invalid backends to HAProxy, it won't start!
+
+- Implement a workaround among these two:
+
+  - remove invalid backends from the list before starting HAProxy
+
+  - wait until all backends are valid before starting HAProxy
+
+---
+
+## Goal 3
+
+- We'd like HAProxy to pick up ConfigMap updates automatically
+
+- How can we do that?
+
+---
+
+## Hints
+
+- Check the following slides if you need help!
+
+--
+
+- We want to generate the HAProxy configuration in an `initContainer`
+
+--
+
+- The `namespaces` entry of the `rainbow` ConfigMap should be exposed to the `initContainer`
+
+--
+
+- The HAProxy configuration should be in a volume shared with HAProxy