vagrant-smartos-zones plugin check and install

With just vagrant installed, starting from the vagrantcloud box [smartos-base64-13.4.0](https://vagrantcloud.com/livinginthepast/smartos-base64-13.4.0):
```
$ vagrant init livinginthepast/smartos-base64-13.4.0
[...]
$ vagrant up
[...]
Vagrant:
* Unknown configuration section 'global_zone'.
```
This commit modifies the box Vagrantfile to check for the plugin and install it using a subshell. 
```
lab $ vagrant up
vagrant-smartos-zones plugin is missing. Installing...
+ vagrant plugin install vagrant-smartos-zones
Now try again.
```
Vagrant currently lacks box/plugin dependencies, but apparently this [will be addressed](https://github.com/mitchellh/vagrant/issues/1874#issuecomment-27560440), in which case this should be reviewed.

/cc @sax, @khushil