Autodetect K3s cluster flavor

[tklauser]

The first commit removes an unnecessary always-nil error return when auto-detecting the cluster flavor. The second commit add support for auto-detecting K3s clusters. Example cilium install output:

$ cilium install
🔮 Auto-detected Kubernetes kind: K3s
ℹ️  using Cilium version "v1.11.2"
🔮 Auto-detected cluster name: default
🔮 Auto-detected IPAM mode: cluster-pool
ℹ️  helm template --namespace kube-system cilium cilium/cilium --version 1.11.2 --set cluster.name=default,ipam.mode=cluster-pool,kubeProxyReplacement=disabled,operator.replicas=1,serviceAccounts.cilium.name=cilium,serviceAccounts.operator.name=cilium-operator
🔑 Created CA in secret cilium-ca
🔑 Generating certificates for Hubble...
🚀 Creating Service accounts...
🚀 Creating Cluster roles...
🚀 Creating ConfigMap for Cilium version 1.11.2...
🚀 Creating Agent DaemonSet...
level=warning msg="spec.template.spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution.nodeSelectorTerms[1].matchExpressions[0].key: beta.kubernetes.io/os is deprecated since v1.14; use \"kubernetes.io/os\" instead" subsys=klog
level=warning msg="spec.template.metadata.annotations[scheduler.alpha.kubernetes.io/critical-pod]: non-functional in v1.16+; use the \"priorityClassName\" field instead" subsys=klog
🚀 Creating Operator Deployment...
⌛ Waiting for Cilium to be installed and ready...
♻️  Restarting unmanaged pods...
♻️  Restarted unmanaged pod kube-system/helm-install-traefik-crd--1-gxtmd
♻️  Restarted unmanaged pod kube-system/helm-install-traefik--1-j9pr6
♻️  Restarted unmanaged pod kube-system/local-path-provisioner-84bb864455-v4z6f
♻️  Restarted unmanaged pod kube-system/metrics-server-ff9dbcb6c-6stf6
♻️  Restarted unmanaged pod kube-system/coredns-96cc4f57d-f7kcl
✅ Cilium was successfully installed! Run 'cilium status' to view installation health

[ldelossa]

LGTM