Providing backward compatibility to instantiate NetworkcacheClient.

Well, the new version of slapos.libnetworkcache has being developed,
and we need to provided backward compatibility to the old versions.

So, slapos.buildout must be able to instantiate a NetworkcacheClient
object even if an TypeError is raised.

src/zc/buildout/networkcache.py

@@ -74,7 +74,7 @@ def download_network_cached(dir_url, cache_url, path, url, logger,
 
     logger.info('Downloading %s from network cache.' % url)
     try:
-        nc = NetworkcacheClient(shacache=cache_url, shadir=dir_url,
+        nc = getNetworkcacheClient(shacache=cache_url, shadir=dir_url,
                signature_private_key_file=signature_private_key_file,
                signature_certificate_file_list=signature_certificate_file_list)
         file_descriptor = nc.select(directory_key)
@@ -117,7 +117,7 @@ def upload_network_cached(dir_url, cache_url, external_url, path, logger,
 
     f = open(path, 'r')
     try:
-        nc = NetworkcacheClient(shacache=cache_url, shadir=dir_url,
+        nc = getNetworkcacheClient(shacache=cache_url, shadir=dir_url,
                signature_certificate_file_list=signature_certificate_file_list,
                signature_private_key_file=signature_private_key_file)
         return nc.upload(f, directory_key, **kw)
@@ -144,4 +144,18 @@ def get_filename_from_url(url):
     return name
 
 
+def getNetworkcacheClient(shacache, shadir, signature_private_key_file,
+                          signature_certificate_file_list):
+  """
+    Provides backward compatibility to instantiate NetworkcacheClient.
+  """
+  try:
+    nc = NetworkcacheClient(shacache, shadir, signature_private_key_file,
+                            signature_certificate_file_list)
+  except TypeError:
+    nc = NetworkcacheClient(shacache, shadir)
+
+  return nc
+
+
 from download import check_md5sum