Add e2e test for CNI with Cilium

[stefanprodan]

This PR adds an e2e test to verify that Flux can deploy a CNI plugin.

Test workflow:

• Install Kubernetes Kind without a CNI plugin
• Install Flux on the cluster
• Test that the Flux pod starts without a CNI plugin
• Install Cilium using a Flux HelmRelease
• Wait for Flux to deploy Cilium
• Test that CoreDNS has become healthy

Followup: #8