'Add an empty storage group to a parent storage group.
This method adds an existing storage group to another storage
group, i.e. cascaded storage groups.
:param array: the array serial number
:param child_sg: the name of the child sg
:param parent_sg: the name of the parent sg
:param extra_specs: the extra specifications'

def add_empty_child_sg_to_parent_sg(self, array, child_sg, parent_sg, extra_specs):

    payload = {'editStorageGroupActionParam': {'addExistingStorageGroupParam': {'storageGroupId': [child_sg]}}}
    (sc, job) = self.modify_storage_group(array, parent_sg, payload, version='83')
    self.wait_for_job('Add    child    sg    to    parent    sg', sc, job, extra_specs)
