Change the default value for FeatureType and Coverage in resource.py

Currently, both the 'enabled' and 'advertised' properties for resource FeatureType and Coverage return string values of 'true' or 'false' rather than boolean. Not sure why this is, but setting the default value to a boolean creates problems with GeoNode updatelayers functionality, which expects the string types. Either GeoNode geonode/geoserver/helpers.py needs to be updated (possibly other locations as well), or the default value here needs to be changed.