Add late function execution in schema

[godfryd]

Eg. Instead of struct in jobs field put a function invocation in string. Example:

def get_jobs(ctx):
    ....
    return jobs

def stage(ctx):
    return {
        "parent": "root",
        "triggers": {
            "parent": True,
        },
        "parameters": [],
        "configs": [],
        "jobs": get_jobs
    }

get_job is invoked with ctx when it is needed.