Modify catalog.get_resource so it doesn't have to retrieve list of all resources in a store

Modified the catalog.get_resource() function to avoid getting a list of all resources in a feature datastore, because thousands of resources could potentially all share the same PostGIS store.

Also, fixed a test (testCreatePostgresLayer).

If this PR is accepted, some integration tests for GeoNode will have to be changed, for example,

from:
        ```self.assertRaises(FailedRequestError, lambda: gs_cat.get_resource(shp_layer.name, store=shp_store))```


to:
    ```self.assertEquals(None, gs_cat.get_resource(shp_layer.name, store=shp_store))```