DSL for tasks does not restrict content of on-success, on-error, on-complete

The DSL for tasks does not restrict the content of the on-success, on-error, on-complete statements but defines them as types.ANY:

class DirectWorkflowTaskSpec(TaskSpec):
    _polymorphic_value = 'direct'

    _direct_workflow_schema = {
        "type": "object",
        "properties": {
            "type": {"enum": [_polymorphic_value]},
            "join": {
                "oneOf": [
                    {"enum": ["all", "one"]},
                    types.POSITIVE_INTEGER
                ]
            },
            "on-complete": types.ANY,
            "on-success": types.ANY,
            "on-error": types.ANY
        }
    }

This makes it easy to introduce bugs into the workflow which are difficult to find, for example:

on-success:
  - my_next_task '{{ _.test1 != None }}'

is valid according to the schema but it does not work the way it looks, because the Jinja condition is NOT executed and the my_next_task task is always executed, even when _.test1 is None.

After spending way too many hours trying to track down errors like this I think it would be better to specify the exact syntax that is allowed for on-success/on-error/on-complete rather than allowing anything.

The fix for this bug will define the specific types that are allowed for these branching statements.