Cassandra/Scylla replication factor and replication strategy must be provided and frozen when creating/deploying the cluster

[matt-mazzucato]

Astarte keyspace is created once housekeeping starts and cannot be further modified. This must be taken into account when considering both replication strategy and replication factor.
The following facts must be considered:

• the naming of the env variables related to the replication strategy must reflect that their value is set only once at startup (e.g. prepend bootstrap or initial to env);
• currently, only Simple Strategy is supported. Network Topology Strategy should be supported (or enforced);
• a proper data structure should be defined in order to define replication strategy and replication factor env. A first proposal consists of two alternatives: (1) a properly formatted string, (2) a valid json object. Further alternatives can be discussed.

Related to the aforementioned points, on the operator's end, the following facts hold:

• the replication strategy/factor should be defined as required in the crd;
• an admission webhook might be employed to prevent the user from modifying the replication strategy/factor once the resource is created.