Check SSH key permissions in machine.ssh_info

@mitchellh while working on #4652, I thought that it might be better to apply `check_key_permissions` in single place. That will avoid duplication and ensure that this best effort is always applied.

Notes:
- With this change, something like `vagrant ssh-config` will now also fix the private key permissions (if needed)
- I am totally unsure if `machine.ssh_info` can be memoized (as proposed in 89a4a29). Is there some VM provider that can change these SSH information during the execution of the same Vagrant command?

//cc @julienvey