Add 'cluster config-template' command to generate the default cluster configuration

[innobead]

Is your feature request related to a problem? Please describe.
Have a command to generate the default cluster configuration for the declarative cluster configuration purpose.

Describe the solution you'd like
A clear and concise description of what you want to happen.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[innobead]

➜  kubefire (master) ✗ ./target/kubefire cluster config-template            
name: ""
bootstrapper: kubeadm
pubkey: ""
prikey: ""
version: ""
image: ghcr.io/innobead/kubefire-opensuse-leap:15.2
kernel_image: ghcr.io/innobead/kubefire-ignite-kernel:4.19.125-amd64
kernel_args: console=ttyS0 reboot=k panic=1 pci=off ip=dhcp security=apparmor apparmor=1
extra_options: {}
deployed: false
master:
  count: 1
  memory: 2GB
  cpus: 2
  disk_size: 10GB
worker:
  count: 0
  memory: 2GB
  cpus: 2
  disk_size: 10GB

[innobead]

➜  kubefire (master) ✔ ./target/kubefire cluster config demo-kubeadm -o yaml
name: demo-kubeadm
bootstrapper: kubeadm
pubkey: /home/davidko/.kubefire/clusters/demo-kubeadm/key.pub
prikey: /home/davidko/.kubefire/clusters/demo-kubeadm/key
version: v1.19.1
image: ghcr.io/innobead/kubefire-opensuse-leap:15.2
kernel_image: ghcr.io/innobead/kubefire-ignite-kernel:4.19.125-amd64
kernel_args: console=ttyS0 reboot=k panic=1 pci=off ip=dhcp security=apparmor apparmor=1
extra_options:
  api_server_options:
  - audit-log-maxage=10
  controller_manager_options:
  - concurrent-deployment-syncs=10
  - concurrent-endpoint-syncs=8
  scheduler_options:
  - kube-api-burst=101
deployed: true
master:
  count: 1
  memory: 2GB
  cpus: 2
  disk_size: 10GB
worker:
  count: 0
  memory: 2GB
  cpus: 2
  disk_size: 10GB