Add a geonode_type field in ResourceBase model

[capooti]

As discussed here:
#1347

I propose to add a geonode_type field in ResouceBase (after releasing GeoNode 2.0), in order to populate it with the corresponding values (layer, map, document) when instantiating the object.
This way we will have a clear way to query ResourceBase, without getting the information in the ugly way we currently do (https://github.com/GeoNode/geonode/blob/master/geonode/base/models.py#L306).

[simod]

We have plans to use django-polymorphic that will solve this. Let's wait untile the api work will be in master then close this.

[simod]

closing as should be solved by polymorphic