ssh: validate the ssh-key parameter if given

[afbjorklund]

Fixes the old (2016) TODO* from docker-machine, and might help to troubleshoot #10289

* https://github.com/docker/machine/blob/bd45ab13d88c32a3dd701485983354514abc41fa/drivers/generic/generic.go#L114

BEFORE:

🤦 StartHost failed, but will try again: creating host: create: creating: usermod: NewSession: new client: new client: Error creating new native config from ssh using: vagrant, &{[] [/home/anders/.minikube/machines/minikube/bogus]}: ssh: no key found

AFTER:

precreate: SSH key does not parse: "/tmp/bogus": ssh: no key found

[afbjorklund]

Here are the current errors, if something unexpected happens while reading the key file:

🤦  StartHost failed, but will try again: creating host: create: precreate: open /tmp/inaccessible: permission denied
😿  Failed to start ssh bare metal machine. Running "minikube delete" may fix it: creating host: create: precreate: open /tmp/inaccessible: permission denied

❌  Exiting due to GUEST_PROVISION: Failed to start host: creating host: create: precreate: open /tmp/inaccessible: permission denied

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: afbjorklund, medyagh

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