[Feature] ipsec for Flannel and k3sup install

[alexellis]

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This feature enables the ipsec backend for Flannel in the k3sup install command

Expected Behaviour

An additional flag to k3sup install should enable ipsec for Flannel as per Darren's comment (@ibuildthecloud) here -> https://twitter.com/ibuildthecloud/status/1200119925500592129

Current Behaviour

k3s cannot be used on Packet.com due to the way flannel has no authentication and Packet.com has no firewall available.

Possible Solution

• Add the flag to the install.go file.
• Work out what parameter is required in the installer script and make that change
• Check that agents can still join in this mode

https://rancher.com/docs/k3s/latest/en/installation/install-options/ https://github.com/rancher/k3s/issues/951

[alexellis]

/lock: stale