Remove default values on node configuration

Currently, some default values are set on a node which leads to have misleading beavhiors when workflow are deployed.

• Do not set a default value due date expression. Make sure the question mark help gives a sample value "CurrentDate.days(5)"
• Do not set a default value for "Compute additional assignees", make sure the sample is available in the question mark: "NodeVariables["sample_variable"]"
• Rename "Compute additional assignees by "Assignees expression" (this makes it consistent with "due date expression"