PEP8 cleanup on node/backuptool.py through node/dht.py

The first of a few minor commits cleaning up style.  I'm trying to familiarize myself with the codebase while still being vaguely useful.
One way to avoid double-checking is to use exceptions:
```python
try:
    existing_index['notaries'].remove(value_json['notary_index_remove'])
except ValueError:
    return
else:
    value = existing_index
```