'Mark a cgsnapshot as available for deletion from the backend.
E-Series snapshots cannot be deleted out of order, as older
snapshots in the snapshot group are dependent on the newer
snapshots. A "soft delete" results in the cgsnapshot being removed
from Cinder management, with the snapshot marked as available for
deletion once all snapshots dependent on it are also deleted.
:param es_cg: E-Series consistency group
:param snap_seq_num: unique sequence number of the cgsnapshot
:return: an update to the snapshot index'

def _soft_delete_cgsnapshot(self, es_cg, snap_seq_num):

    index = self._get_soft_delete_map()
    cg_ref = es_cg['id']
    if (cg_ref in index):
        bitset = na_utils.BitSet(int(index[cg_ref]))
    else:
        bitset = na_utils.BitSet(0)
    seq_nums = set([snap['pitSequenceNumber'] for snap in self._client.get_consistency_group_snapshots(cg_ref)])
    for (i, seq_num) in enumerate(sorted(seq_nums)):
        if (snap_seq_num == seq_num):
            bitset.set(i)
            break
    index_update = self._cleanup_cg_snapshots(cg_ref, seq_nums, bitset)
    self._merge_soft_delete_changes(index_update, None)
    return (None, None)
