Setting cassandra_hosts lead to an error

[Benjamin Allot]

When setting cassandra_hosts, the formatting is wrong in the db_settings.py file

```
cassandra_hosts = ['10.141.24.230, 10.141.24.201, 10.141.24.28, 10.141.24.35, 10.141.24.139']
```

you can see here that the list of IPs are all between the same quotes with the following juju config

```
juju config errors-app daisy-cassandra-host='10.141.24.230 10.141.24.201 10.141.24.28 10.141.24.35 10.141.24.139'
```

Also, the charm leads to an error when there is no relation but cassandra_hosts are set but this seems already addressed in revno 137: https://bazaar.launchpad.net/~daisy-pluckers/charms/xenial/errors/trunk/revision/137