Workflow creation and assignment definition

As of HG1705 CMSWEB cycle, we've tighten the request spec validation for both creation and assignment. In summary they are:

• there is a specific set of parameters that must be provided during request creation (optional=False). There are also optional parameters that have a default value in ReqMgr;
• there is a specific set of parameters that must be provided during request assignment (assign_optional=False). There are also optional parameters that have a default value in ReqMgr;
• unknown arguments (not defined in the spec files) are no longer accepted/removed and the request will fail validation. Moreover, arguments are case-sensitive;
• there are a few arguments that are allowed both at creation and at assignment. Besides those, you must provide only creation or only assignment parameters, according to the operation you want to accomplish.
• inner Task/Step dictionary (for TaskChain/StepChain) are also validated.