Fix OpenSearch upgrade tasks idempotency

Shard allocation is disabled at the start of the OpenSearch upgrade
task. This is set as a transient setting, meaning it will be removed
once the containers are restarted. However, if there is not change in
the OpenSearch container it will not be restarted so the cluster is left
in a broken state: unable to allocate shards.

This patch moves the pre-upgrade tasks to within the handlers, so shard
allocation and the flush are only performed when the OpenSearch
container is going to be restarted.

Closes-Bug: #2049512
Change-Id: Ia03ba23bfbde7d50a88dc16e4f117dec3c98a448