Spaces in ssh key file names prevent them from being copied into the VM

[lyondhill]

From @sanderson on December 5, 2016 15:23

This was found by @danhunsaker which helped troubleshoot a problem @atourino ran into:

https://github.com/nanobox-io/nanobox/blob/master/generators/hooks/build/user.go#L15

Not sure the best way to make Ruby properly quote shell commands inside the VM, but that'll be the actual fix

Copied from original issue: nanobox-io/nanobox#282