RAPI: fix a pylint warning

Child classes of _R_TAGS must define TAG_LEVEL, but for good style let's
define it also here to at least ensure we don't get a 'Unknown
attribute' exception.

Of course, this also silences a pylint warning.

Reviewed-by: amishchenko

lib/rapi/rlib2.py

@@ -459,6 +459,7 @@ class _R_Tags(baserlib.R_Generic):
   TAG_LEVEL for it.
 
   """
+  TAG_LEVEL = None
 
   def __init__(self, items, queryargs, req):
     """A tag resource constructor.