make shacache related options optional.

src/zc/buildout/easy_install.py

@@ -906,10 +906,13 @@ class Installer:
                and globals().get('_upload_dir_url'):
                 upload_network_cached(_upload_dir_url,
                    _upload_cache_url, dist.location, new_location, logger,
-                   _signature_private_key_file,
-                   _shacache_ca_file, _shacache_cert_file,
-                   _shacache_key_file, _shadir_ca_file,
-                   _shadir_cert_file, _shadir_key_file)
+                   globals().get('_signature_private_key_file'),
+                   globals().get('_shacache_ca_file'),
+                   globals().get('_shacache_cert_file'),
+                   globals().get('_shacache_key_file'),
+                   globals().get('_shadir_ca_file'),
+                   globals().get('_shadir_cert_file'),
+                   globals().get('_shadir_key_file'))
 
         if (download_cache
             and (realpath(new_location) == realpath(dist.location))