fix uninitialized constant error for gentoo plugin

While trying to 

```
vagrant up
```

a gentoo vm I got the following error:

```
/Applications/Vagrant/embedded/gems/gems/vagrant-1.2.2/plugins/guests/gentoo/plugin.rb:23:in `block in <class:Plugin>': uninitialized constant VagrantPlugins::GuestGentoo::Cap::ConfigureNetworks (NameError)
```

I then had a look at the gentoo plugin and found out that indeed GuestGentoo was not defined. After applying the changes in pull request the vm did start without errors.