SSHKeyPath is not being set for the machine

[fgrehm]

Not sure if it is a requirement for docker-machine drivers but it seems that the xhyve driver is not setting it!

$ docker-machine create remove --driver xhyve
....
Docker is up and running!
To see how to connect your Docker Client to the Docker Engine running on
this virtual machine, run: docker-machine env remove

$ docker-machine inspect remove | grep -i sshkey
        "SSHKeyPath": "",

I have not been able to check other drivers besides virtualbox (which sets up that field). LMK if you need any additional information!

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References https://github.com/codekitchen/dinghy/pull/150 /cc @codekitchen