Do not check for file if variable is None.

slapos/grid/slapgrid.py

@@ -180,6 +180,7 @@ def parseArgumentTupleAndReturnSlapgridObject(*argument_tuple):
     mandatory_file_list.append(signature_private_key_file)
 
   for f in mandatory_file_list:
+    if f is not None:
       if not os.path.exists(f):
         parser.error('File %r does not exists.' % f)