BACKWARD INCOMPATIBLE CHANGE: use signature-private-key-file option

Use same option name as libnetworkcache so that config files are compatible

slapcache/signature.py

@@ -65,7 +65,7 @@ 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')
+    self.signature_private_key_file = get_('signature-private-key-file')
     self.shacache_ca_file = get_('shacache-ca-file')
     self.shacache_cert_file = get_('shacache-cert-file')
     self.shacache_key_file = get_('shacache-key-file')