The upload-authentication-file comes from networkcache section.

Well, according to this commit at slapos.core.git:
  - 017a02ef202b69dff03e85de18293bc30ac6b545

This parameter is always under networkcache section to keep the
configuration consistenty.

src/zc/buildout/buildout.py

@@ -358,6 +358,8 @@ class Buildout(UserDict.DictMixin):
                     'upload-cache-url', '')
                 self.upload_dir_url = networkcache_section.get(
                     'upload-dir-url', '')
+                self.upload_authentication_file = networkcache_section.get(
+                    'upload-authentication-file', '')
                 self.signature_certificate_file = networkcache_section.get(
                     'signature-certificate-file', '')
                 self.signature_private_key_file = networkcache_section.get(
@@ -373,6 +375,7 @@ class Buildout(UserDict.DictMixin):
                 options['__networkcache__download-dir-url'] = self.download_dir_url
                 options['__networkcache__upload-cache-url'] = self.upload_cache_url
                 options['__networkcache__upload-dir-url'] = self.upload_dir_url
+                options['__networkcache__upload-authentication-file'] = self.upload_authentication_file
                 options['__networkcache__signature-certificate-file'] = self.signature_certificate_file
                 options['__networkcache__signature-private-key-file'] = self.signature_private_key_file