Create a PluginConfigData as polymorphic model

[pablodgonzalez]

class PluginConfigData(PolymorphicModel):

The idea about this models to get the chance to plugins developer to add a custom config model to use for the plugin itself. They can be now registering the model to admin site.
IT is just to clarify the posibility and avoid add more environmental setting or plugins, and just config in the UI.