[utils] Add asset_type and asset_folder to Storage

This will centralize the information and avoid duplicating
strings everywhere when one needs data from one of the various
assets.

beat/backend/python/utils.py

@@ -219,8 +219,16 @@ class File(object):
 class Storage(object):
     """Resolves paths for objects that provide only a description"""
 
+    asset_type = None
+    asset_folder = None
+
     def __init__(self, path):
 
+        if not all(
+            [type(attr) == str for attr in [self.asset_type, self.asset_folder]]
+        ):
+            raise TypeError("asset_type and asset_folder must be configure properly")
+
         self.path = path
         self.json = File(self.path + ".json")
         self.doc = File(self.path + ".rst")