Update snapshots dict with current snapshots

    dataset: string
        name of filesystem or volume
    prefix : string
        prefix for the snapshots
        e.g. 'test' will result in snapshots being named 'test-yyyymmdd_hhmm'
    snapshots : OrderedDict
        preseeded OrderedDict with configuration

def _schedule_snapshot_retrieve(dataset, prefix, snapshots):
    '''
    Update snapshots dict with current snapshots

    dataset: string
        name of filesystem or volume
    prefix : string
        prefix for the snapshots
        e.g. 'test' will result in snapshots being named 'test-yyyymmdd_hhmm'
    snapshots : OrderedDict
        preseeded OrderedDict with configuration

    '''
    ## NOTE: retrieve all snapshots for the dataset
    for snap in sorted(__salt__['zfs.list'](dataset, **{'recursive': True, 'depth': 1, 'type': 'snapshot'}).keys()):
        ## NOTE: we only want the actualy name
        ##       myzpool/data@zbck-20171201_000248 -> zbck-20171201_000248
        snap_name = snap[snap.index('@')+1:]

        ## NOTE: we only want snapshots matching our prefix
        if not snap_name.startswith('{0}-'.format(prefix)):
            continue

        ## NOTE: retrieve the holds for this snapshot
        snap_holds = __salt__['zfs.holds'](snap)

        ## NOTE: this snapshot has no holds, eligable for pruning
        if not snap_holds:
            snapshots['_prunable'].append(snap)

        ## NOTE: update snapshots based on holds (if any)
        ##       we are only interested in the ones from our schedule
        ##       if we find any others we skip them
        for hold in snap_holds:
            if hold in snapshots['_schedule'].keys():
                snapshots[hold].append(snap)

    return snapshots