Ticket 50 - Add ns_job_set_data helper
    
    Bug Description:  Part way through the use of a job, a caller may wish to
    change the contents of the job private data. A function should be available
    to allow this.
    
    Fix Description:  Add the ns_job_set_data function. Add cmocka test structure
    to show how the set data works, and prove it's correctness.
    
    https://fedorahosted.org/nunc-stans/ticket/50
    
    Author: wibrown
    
    Review by: rmeggins (Thanks!)