Fix tls-san handling in rke2 config

[ghsbhatia]

This pull request addresses the following issue:

#93

The fix has been tested on ubuntu 20.04 for the following scenarios:

A) tls-san is present in rke2-config parameter

# Server Configuration
write-kubeconfig-mode: "0644"
tls-san:
  - k8s.foobar.com
node-label:
  - "name=server"
  - "os=ubuntu"
kube-controller-manager-arg:
  - "bind-address=0.0.0.0"
kube-scheduler-arg:
  - "bind-address=0.0.0.0"
node-taint:
  - "CriticalAddonsOnly=true:NoExecute"

The generated /etc/rancher/rke2/config.yaml is:

# Additional user defined configuration
# Server Configuration
write-kubeconfig-mode: "0644"
tls-san:
  - foobar-rke2-wkd-rke2-cp-808cf9ad440d5fe8.elb.us-east-1.amazonaws.com
  - k8s.foobar.com
node-label:
  - "name=server"
  - "os=ubuntu"
kube-controller-manager-arg:
  - "bind-address=0.0.0.0"
kube-scheduler-arg:
  - "bind-address=0.0.0.0"
node-taint:
  - "CriticalAddonsOnly=true:NoExecute"
token: HeHFQYLVfVykWWvrHzD3bqrkyUOVG726TpqwIXNG
cloud-provider-name: "aws"

B) tls-san is not specified in rke2-config parameter

# Server Configuration
write-kubeconfig-mode: "0644"
node-label:
  - "name=server"
  - "os=ubuntu"
kube-controller-manager-arg:
  - "bind-address=0.0.0.0"
kube-scheduler-arg:
  - "bind-address=0.0.0.0"
node-taint:
  - "CriticalAddonsOnly=true:NoExecute"

The generated /etc/rancher/rke2/config.yaml is:

# Additional user defined configuration
# Server Configuration
write-kubeconfig-mode: "0644"
node-label:
  - "name=server"
  - "os=ubuntu"
kube-controller-manager-arg:
  - "bind-address=0.0.0.0"
kube-scheduler-arg:
  - "bind-address=0.0.0.0"
node-taint:
  - "CriticalAddonsOnly=true:NoExecute"
token: HeHFQYLVfVykWWvrHzD9bqrkyUOVH726TpqwIXNG
cloud-provider-name: "aws"
tls-san:
 - foobar-rke2-ut1-rke2-cp-ffa416fcd96b7f24.elb.us-gov-west-1.amazonaws.com

[adamacosta]

Deployed with a tls-san set and without. Both worked, handled correctly.