Update related-object methods in DatasetCommit to use parent_job_model

[davecap]

if self['parent_job_model'] == 'datasets.DatasetImport':  
       self._client.DatasetImport.retrieve(self['parent_job_id'])

do we want to add other possible methods in here also?

def dataset_migration(self):
    # returns its parent migration
    pass

def parent_commit(self):
   # if this a rollback, returns its parent 
   pass