NetworkcacheClient must use libconnection

To download contents in a correct way, it must use the
libnetworkcache.

slapos/libnetworkcache.py

@@ -22,6 +22,7 @@ import tempfile
 import urllib
 import urllib2
 import M2Crypto
+import slapos.libconnection
 from slapos.libnetworkcacheutils import parseUrl
 
 
@@ -202,7 +203,7 @@ class NetworkcacheClient(object):
         return True
 
     for certificate_url in self.signature_certificate_url_list:
-      file_descriptor = self._fetchCertificateFileFromUrl(certificate_url)
+      file_descriptor = slapos.libconnection.download(certification_file_url)
       try:
         file_name = file_descriptor.name
         if self._verifySignatureCertificate(signature_string, file_name):
@@ -221,14 +222,6 @@ class NetworkcacheClient(object):
     VerifyEVP.verify_update('')
     return VerifyEVP.verify_final(signature_string.decode('base64'))
 
-  def _fetchCertificateFileFromUrl(self, certification_file_url):
-    """ Download the certification files from the url. """
-    file_descriptor = tempfile.NamedTemporaryFile()
-    path, headers = urllib.urlretrieve(certification_file_url, file_descriptor.name)
-    file_descriptor.seek(0)
-    return file_descriptor
-
-
 
 class DirectoryNotFound(Exception):
   pass