`Vagrant.has_plugin?` should use the gem name

As discussed in the comments of #2189, for consistency `Vagrant.has_plugin?` should probably use gem names instead of the `name` attribute of the plugins. The gem name is already used for example in `vagrant plugin install/list` and `Vagrant.require_plugin`. 

Also it requires extra effort to search the name attribute. And the plugin authors might even change it between releases.