[ClickHouse] Enable replicated mode once Nexus provisions services

[karencfv]

ClickHouse replicated functionality has been implemented in #3494. This functionality had to be disabled for the following reasons:

• RSS, which provisions services, only runs once during rack set up.
• We already had customer racks running single node CH.
• We want as little as possible divergence between racks, and this functionality did not warrant such divergence.

Once #732 has been implemented, replicated mode should be re-enabled in the following manner:

• The method_script.sh for the clickhouse service is set to run single node mode by default, but can be switched to run on replicated mode by swapping a variable to false. When we migrate all racks to a replicated ClickHouse setup, all logic related to running on single node will be removed from that file.
• The number of zones defined through RSS will stay the same. Instructions on how to tweak them to launch in replicated mode have been left in the form of comments.

Solely enabling this way can incur in data loss due to the tables using different engines (e.g. MergeTree vs ReplicatedMergeTree), and not having been set up within a cluster. If we decide that this is not acceptable, this plan should also include a way to migrate data over.

[andrewjstone]

@karencfv I think we can safely close this now that we are in a reconfigurator based world. The instructions don't seem to be valid anymore.

[karencfv]

Yup! makes sense