'Override any block devices from the first list by device name
:param overridable_mappings: list which items are overridden
:param overrider_mappings: list which items override
:returns: A merged list of bdms'

def _merge_bdms_lists(self, overridable_mappings, overrider_mappings):

    device_names = set((bdm['device_name'] for bdm in overrider_mappings if bdm['device_name']))
    return (overrider_mappings + [bdm for bdm in overridable_mappings if (bdm['device_name'] not in device_names)])
