The signature-private-key-file comes from networkcache section.

Well, according to the commit d8dca883a54ee87aafed3a82921594e5b508b388
at slapos.core.git, this parameter is always under networkcache section
to keep the configuration consistenty.

src/zc/buildout/buildout.py

@@ -360,7 +360,7 @@ class Buildout(UserDict.DictMixin):
                     'upload-dir-url', '')
                 self.signature_certificate_file = networkcache_section.get(
                     'signature-certificate-file', '')
-                self.signature_private_key_file = options.get(
+                self.signature_private_key_file = networkcache_section.get(
                     'signature-private-key-file', '')
 
                 self._logger.info('Networkcache enabled.')