Use signature public file instead of private file.

The signature private file is not required to verify the signature it is
only used to create the signature string.

The signature verification requires only the public file...

slapos/libnetworkcache.py

@@ -169,7 +169,7 @@ class NetworkcacheClient(object):
                'Directory: %s. Entries: %s.' % (directory_key, str(data_list)))
 
     sha512 = None
-    if self.signature_private_file not in _MARKER:
+    if self.signature_public_file not in _MARKER:
       for information_dict, signature in data_list:
         if self._verifySignature(signature):
           sha512 = information_dict.get('sha512')