Revoved useless import and fix typo.

slapos/libnetworkcache.py

@@ -13,7 +13,6 @@
 ##############################################################################
 
 
-import base64
 import hashlib
 import httplib
 import json
@@ -203,7 +202,7 @@ class NetworkcacheClient(object):
         return True
 
     for certificate_url in self.signature_certificate_url_list:
-      file_descriptor = slapos.libconnection.download(certification_file_url)
+      file_descriptor = slapos.libconnection.download(certificate_url)
       try:
         file_name = file_descriptor.name
         if self._verifySignatureCertificate(signature_string, file_name):