pluto: don't error on missing settings

[etungsten]

Issue number:
N/A

Description of changes:

Author: Erikson Tung <etung@amazon.com>
Date:   Thu Oct 21 10:48:44 2021 -0700

    pluto: don't error on missing settings
    
    This makes it so that pluto does not error early if some Bottlerocket
    settings are missing to allow fallbacks later when determining node-ip
    and cluster-dns-ip.

Testing done:
Was able to launch a Bottlerocket instance with kubernetes.standalone-mode and the host comes up fine and kubelet is running fine with a node-ip:

bash-5.0# systemctl status kubelet
● kubelet.service - Kubelet
     Loaded: loaded (/x86_64-bottlerocket-linux-gnu/sys-root/usr/lib/systemd/system/kubelet.service; enabled; vendor preset: enabled)
    Drop-In: /etc/systemd/system/kubelet.service.d
             └─exec-start.conf
     Active: active (running) since Thu 2021-10-21 18:09:10 UTC; 28s ago
...
     CGroup: /runtime.slice/kubelet.service
             └─1581 /usr/bin/kubelet --cloud-provider  --config /etc/kubernetes/kubelet/config --container-runtime=remote --container-runtime-endpoint=unix:///run/dockershim.sock --containerd=/run/dockershim.sock --network-plugin cni --root-dir /var/lib/kubelet --cert-dir /var/lib/kubelet/pki --node-ip 192.168.26.35 --node-labels  --register-with-taints  --pod-infra-container-image 602401143452.dkr.ecr.us-west-2.amazonaws.com/eks/pause-amd64:3.1

bash-5.0# cat /etc/kubernetes/kubelet/config 
---
kind: KubeletConfiguration
apiVersion: kubelet.config.k8s.io/v1beta1
address: 127.0.0.1
authentication:
  anonymous:
    enabled: true
  webhook:
    enabled: false
authorization:
  mode: AlwaysAllow

Repeated this test for combinations of standalone-mode = true and other kubernetes settings being set. The results are the same.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[etungsten]

Push above addresses @tjkirch 's comments.