[cassandra] Configure the monitoring of the cluster

A prometheus is needed to compare the different benchmarks.

In order to not impact the cassandra nodes, the etcd/controle plane nodes will received the worker role too. Only the monitoring workload will be deployed on the nodes.

Too allow HA, the number of prometheus replicas will be set to 3 and the data location in the same directory on all the nodes. The dataretention will be set to 365 to avoid data loss until the "thanosification" is done.

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Migrated from T4389 (view on Phabricator)