Configure cassanda_keyspace from juju config

[Benjamin Allot]

During deployment of a new application, cassandra_keyspace configuration item is missing.

This is the content of the local_config.py file after a brand new deployment

```
swift_bucket = 'daisy-production-cores'
oops_repository = '/srv/daisy.ubuntu.com/production/oops-local'
oops_amqp_host = '10.x.x.x'
oops_amqp_username = 'REDACTED'
oops_amqp_password = 'REDACTED'
oops_amqp_vhost = 'whoopsie-prodstack'
oops_amqp_exchange = 'REDACTED'
oops_amqp_routing_key = 'REDACTED'
statsd_host = '10.x.x.x'
statsd_port = '10021
```

Hoever, there was an issue with that because the cassandra_keyspace item was missing.

Most of the configuration items are going in the juju config and are generated in other files.
They should be able to be set up there.