Store the template and deletion strategy used for managed resources

What does this MR do and why?

Stores the name of the template used to generate Kubernetes resources for an environment, as well as the deletion strategy specified by the template (if present). These will be used to determine the deletion behaviour when the environment is later stopped or removed.

References

#507486 (closed)

How to set up and validate locally

1. Follow the instructions in !177525 (merged) to create a managed resource record.
2. In a Rails console, verify that the template and deletion strategy were persisted with Clusters::Agents::ManagedResource.last.template_name and Clusters::Agents::ManagedResource.last.deletion_strategy.

MR acceptance checklist

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Related to #507486 (closed)