Update snapshots dict with info for a new snapshot

    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_prepare(dataset, prefix, snapshots):
    '''
    Update snapshots dict with info for a new snapshot

    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: generate new snapshot name
    snapshot_create_name = '{dataset}@{prefix}-{timestamp}'.format(
        dataset=dataset,
        prefix=prefix,
        timestamp=datetime.now().strftime('%Y%m%d_%H%M%S')
    )

    ## NOTE: figure out if we need to create the snapshot
    timestamp_now = datetime.now().replace(second=0, microsecond=0)
    snapshots['_create'][snapshot_create_name] = []
    for hold, hold_count in snapshots['_schedule'].items():
        ## NOTE: skip hold if we don't keep snapshots for it
        if hold_count == 0:
            continue

        ## NOTE: figure out if we need the current hold on the new snapshot
        if snapshots[hold]:
            ## NOTE: extract datetime from snapshot name
            timestamp = datetime.strptime(
                snapshots[hold][-1],
                '{0}@{1}-%Y%m%d_%H%M%S'.format(dataset, prefix),
            ).replace(second=0, microsecond=0)

            ## NOTE: compare current timestamp to timestamp from snapshot
            if hold == 'minute' and \
                timestamp_now <= timestamp:
                continue
            elif hold == 'hour' and \
                timestamp_now.replace(**comp_hour) <= timestamp.replace(**comp_hour):
                continue
            elif hold == 'day' and \
                timestamp_now.replace(**comp_day) <= timestamp.replace(**comp_day):
                continue
            elif hold == 'month' and \
                timestamp_now.replace(**comp_month) <= timestamp.replace(**comp_month):
                continue
            elif hold == 'year' and \
                timestamp_now.replace(**comp_year) <= timestamp.replace(**comp_year):
                continue

        ## NOTE: add hold entry for snapshot
        snapshots['_create'][snapshot_create_name].append(hold)

    return snapshots