return firs private ip if no public interfaces are found

This way we can create a 'gateway' machine and connect to it
using ssh.proxy_command

config.vm.define 'gateway' do |cfg|
  cfg.vm.provider :tiktalik do |provider, override|
  provider.networks = [
          "pub1",
          "priv1",
       ]
  end
end

config.vm.define "dc" do |cfg|
  cfg.ssh.proxy_command = "ssh -W %h:%p root@gateway.client.tiktalik.com"
  cfg.vm.provider :tiktalik do |provider, override|
    provider.networks = [
      "priv1"
    ]
  end
end