The default value of this parameter must be None.

The default value was ' ' because it was always being propagating to the
buildout command line. But since the commit
d8dca883, it does not happen anymore.

This option is only propagated to buildout command line if it is not
None.

slapos/grid/slapgrid.py

@@ -97,7 +97,7 @@ def parseArgumentTupleAndReturnSlapgridObject(*argument_tuple):
   parser.add_argument("--key_file", help="SSL Authorisation key file.")
   parser.add_argument("--cert_file",
       help="SSL Authorisation certificate file.")
-  parser.add_argument("--signature_private_key_file", default='',
+  parser.add_argument("--signature_private_key_file", default=None,
       help="Signature private key file.")
   parser.add_argument("--master_ca_file", help="Root certificate of SlapOS "
       "master key.")