Convert node shutdown system property feature flag to setting

[dakrone]

This converts the system property feature flag 'es.shutdown_feature_flag_enabled' to a regular
non-dynamic node setting. This setting can only be set to 'true' on a snapshot build of
Elasticsearch (not a release build).

Relates to #70338

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[gwbrown]

Left one minor suggestion. My only concern is: Are we sure that the tests will pass in release builds? Or do we need something like assumeTrue to prevent the integration tests from running? If we're good on that front, I'm good to merge this.

(admittedly depending on timing we may get rid of the feature flag before the next release is cut, but I'd rather be on the safe side)

[dakrone]

Are we sure that the tests will pass in release builds? Or do we need something like assumeTrue to prevent the integration tests from running?

That's a good point. I think this might cause ES not to start for integration tests also, since we set it to true in build.gradle in a few places. I will investigate this.

[dakrone]

@elasticmachine update branch

[dakrone]

@gwbrown I think this should do the trick, can you take a look?

[gwbrown]

LGTM once CI is happy.

My only question is: Why don't we need assumeTrue or similar in NodeShutdownIT? I'd assume that would fail unless the feature flag is enabled too.

(If it turns out we do, no need for another round of review, that's a very minimal change)

[dakrone]

Why don't we need assumeTrue or similar in NodeShutdownIT? I'd assume that would fail unless the feature flag is enabled too.

Yep, I think we need it there too, I'll add it to all those tests

[probakowski]

@dakrone I've marked it as non-issue, let me know if you think it should be something else