config.smartos.suexec_cmd ignored when set in base Vagrantfile

I can set `config.smartos.suexec_cmd` manually in a downstream Vagrantfile, but when I set this in the base box, Vagrant strangely ignores this and uses the dumb `pfexec` default.

### Vagrant version

Vagrant 2.0.2

### Host operating system

macOS 10.13 High Sierra

### Guest operating system

Haiku nightly hrev hrev51909, using SmartOS Vagrant guest plugin to work around some issues with the Haiku Vagrant plugin

### Vagrantfile (packed into base box by Packer)
```ruby
Vagrant.configure("2") do |config|
config.vm.guest = :smartos
config.ssh.username = "user"
config.smartos.suexec_cmd = "/boot/system/non-packaged/bin/boynamedsu"

config.vm.synced_folder ".", "/boot/vagrant-src", type: "rsync", rsync__args: ["--verbose", "--archive", "--delete", "-zz", "--copy-links"]
end
```

### Expected behavior

On `vagrant up`, the box boots normally.

### Actual behavior

Vagrant runs `pfexec` instead of `boynamedsu`, demonstrating that `config.smartos.suexec_cmd` is ignored.

As a workaround, one can copy and paste `config.smartos.suexec_cmd = "/boot/system/non-packaged/bin/boynamedsu"` into the downstream `Vagrantfile`, though this is tedious. I am planning to work around this behavior by shimming `pfexec` in the base box, so that the default value can resolve to something useful.