uninitialized constant VagrantPlugins::Solidus::Site::Start::SITES_CONFIGS_FILE_HOST_PATH

When using `vagrant site restart`, and the VM is out of ports, this error occurs:

```
$ vagrant site restart
Installing site...
~/.vagrant.d/gems/gems/vagrant-solidus-2.0.1/lib/vagrant-solidus/site/start.rb:23:in `block in execute': uninitialized constant VagrantPlugins::Solidus::Site::Start::SITES_CONFIGS_FILE_HOST_PATH (NameError)
```
