Using _MARKER is overkill.

Following the previous commit, _MARKER is not required anymore.

slapos/libnetworkcache.py

@@ -24,9 +24,6 @@ import urlparse
 import M2Crypto
 
 
-_MARKER = ([], [''], None, '')
-
-
 class NetworkcacheClient(object):
   '''
     NetworkcacheClient is a wrapper for httplib.
@@ -197,7 +194,7 @@ class NetworkcacheClient(object):
     """
       Return the signature based on certification file.
     """
-    if self.signature_private_key_file in _MARKER:
+    if self.signature_private_key_file is None:
       return ''
 
     SignEVP = M2Crypto.EVP.load_key(self.signature_private_key_file)