Setting hostname on OmniOS fails

When attempting to boot an OmniOS Vagrant box [0], the `up` command fails with:

```
[riak1] Setting hostname...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

sudo uname -S riak1
```

It appears that the `-S` option is available for `uname` on SmartOS, but not OmniOS.

[0] http://omnios.omniti.com/media/omnios-latest.box