Return string diff of pipeline definitions.

def _diff(old_pipeline_definition, new_pipeline_definition):
    '''
    Return string diff of pipeline definitions.
    '''
    old_pipeline_definition.pop('ResponseMetadata', None)
    new_pipeline_definition.pop('ResponseMetadata', None)

    diff = salt.utils.data.decode(difflib.unified_diff(
        salt.utils.json.dumps(old_pipeline_definition, indent=4).splitlines(True),
        salt.utils.json.dumps(new_pipeline_definition, indent=4).splitlines(True),
    ))
    return ''.join(diff)