Inform user about dropped key

Otherwise user might spend a lot of time debugging problem of slapcache-upload
misbehaving.

slapcache/signature.py

@@ -65,7 +65,12 @@ class NetworkCache:
     # Not mandatory
     self.dir_url = get_('upload-dir-url')
     self.cache_url = get_('upload-cache-url')
-    self.signature_private_key_file = get_('signature-private-key-file')
+    key_file_key = 'signature-private-key-file'
+    self.signature_private_key_file = get_(key_file_key)
+    key_file_old_key = 'signature_private_key_file'
+    if key_file_old_key in network_cache_info_dict:
+      raise ValueError(
+        '%s is not supported anymore, use %s' % (key_file_old_key, key_file_key))
     self.shacache_ca_file = get_('shacache-ca-file')
     self.shacache_cert_file = get_('shacache-cert-file')
     self.shacache_key_file = get_('shacache-key-file')