move field name constants back to BaseStorage

ffstatus/basestorage.py

@@ -30,6 +30,9 @@ class BaseStorage(object):
     to a file, database, whatever.
     """
 
+    DATAKEY_VPN = '__VPN__'
+    FIELDKEY_UPDATED = '__UPDATED__'
+
     def open(self):
         """
         When overridden in a subclass,

ffstatus/filestorage.py

@@ -12,9 +12,6 @@ from .basestorage import BaseStorage
 class FileStorage(BaseStorage):
     """Provides file-based persistency for BaseStorage"""
 
-    DATAKEY_VPN = '__VPN__'
-    FIELDKEY_UPDATED = '__UPDATED__'
-
     __data = None
 
     def __init__(self, storage_dir):