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
	}

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.