setup: delegate redis flush command

Create a CLI command to *flush redis* in the appropriate module (invenio-app?).

Currently we are using:

``` console
invenio shell --no-term-title -c "import redis; redis.StrictRedis.from_url(app.config['CACHE_REDIS_URL']).flushall(); print('Cache cleared')"
```

As a single command, however this should be delegate and run something similar to `invenio redis flush`.