Load public certificates from config by default

slapos/libnetworkcache.py

@@ -80,6 +80,8 @@ class NetworkcacheClient(object):
       with open(path) as f:
         self.signature_private_key = crypto.load_privatekey(crypto.FILETYPE_PEM,
                                                             f.read())
+    if signature_certificate_list is None:
+      signature_certificate_list = config.get('signature-certificate-list')
     if type(signature_certificate_list) is str:
       # If signature_certificate_list is a string, parse it to a list of
       # certificates