Support existing SSH key pairs

[briandowns]

The Create method generates an SSH key pair for each instance being created. For example, if an operator creates a 3 node Kubernetes cluster using this driver then 3 SSH key pairs will be generated and uploaded to Oxide.

rancher-machine-driver-oxide/oxide.go

Lines 142 to 145 in 8e3b6df

	pubKey, err := d.createSSHKeyPair()
	if err != nil {
	return err
	}

These per-node SSH key pairs remain in Oxide until the Kubernetes node is deleted, creating noise in the Oxide web console and not allowing users to share an SSH key pair across multiple nodes.

Update the driver to use an existing SSH key pair. This will involve accepting an SSH private key as input from the operator and its corresponding SSH public key ID that has already been uploaded into Oxide.

[linear]

SSE-158