add installation of argparse, which is a dependency of the index cleaner

We stumbled upon this bug when setting up the index cleaner. It failed with the error:

```
Traceback (most recent call last):
  File "/opt/logstash/index_cleaner/logstash_index_cleaner.py", line 21, in <module>
    import argparse
ImportError: No module named argparse
```

So here's a fix.