Add a setting to disable stack installed templates

[dakrone]

From #57629 onward we have started installing logs, metrics, and synthetics templates that match the logs-*-*, metrics-*-*, and synthetics-*-* index patterns.

An issue arises when a user upgrades their cluster to a version that includes these templates while also using an index pattern that would coincide with these (such as a user having daily indices named logs-data-things-2020-09-23 for example).

In order to mitigate these issues, we should add a node level setting that disables putting these templates in place automatically. The setting should also be dynamic (so as not to require restarting all the nodes if a user needs to get rid of the templates), and we should also better document the stack plugin, the built in templates, and potential issues that may arrive when a user upgrades.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Features)

[kobelb]

Should we be installing these templates by default? This feels like a breaking change, where users that were previously using Elasticsearch in a legitimate manner by having indices named logs-data-things-2020-09-23 are now unable to upgrade without changing a setting.

[mbarretta]

I've seen this occur already with a metrics-* index template, and yes, it broke their system and forced a rename of the new metrics-*-* template to metrics-kill-9-* (he had a sense of humor). A rename was required because they cannot be deleted.

As opposed to a node-level setting, can something first prompt that a collision will occur and/or force a positive confirmation to proceed? Otherwise, it puts the onus on users to read through the release/upgrade notes, notice the potential collision, and make the setting change (++ dynamic).

Failing that, the change is made and their own index templates will silently fail to apply.

[pugnascotia]

This is awkward for writing deprecation logs to a data stream, since that functionality expects the logs-*-* template to exist and relies on it.

[dakrone]

@pugnascotia we currently already have the setting (#63764 just makes it dynamic), so we cannot remove it, I would suggest maybe that the deprecation log data stream install its own template (with a more narrow index pattern and higher priority) so that it's independent of the stack templates