[ML] Data frame analytics as persistent tasks

[dimitris-athanasiou]

Converts data frame analytics to run as persistent tasks.

Adds the following APIs:

• PUT _ml/data_frame/analysis/{id}
• GET _ml/data_frame/analysis/{id}
• GET _ml/data_frame/analysis/{id}/_stats
• POST _ml/data_frame/analysis/{id}/_start
• DELETE _ml/data_frame/analysis/{id}

[elasticmachine]

Pinging @elastic/ml-core

[dimitris-athanasiou]

I have now updated this PR with some tests. There will definitely be more coming but I believe at this point there's enough to get this merged.

[dimitris-athanasiou]

run elasticsearch-ci/2

[benwtrent]

👍 putting the validations outside of the model

[droberts195]

This isn't needed to delete the configs because the base class will delete the index they're stored in, so they'll all get wiped between tests.

The only thing this is doing is possibly triggering the error that delete throws when you try to delete an analytics process that's running. If that's the reason this method exists then I think it should be renamed to reflect that.

But maybe ESRestTestCase.waitForPendingTasks() already does that check? In which case there is no need to have this method at all.

(As an aside, the anomaly detector and datafeed methods in this class will no longer need to call delete once we're sure every config is in an index, and then those methods can be renamed. But that's for a different PR.)

[benwtrent]

👍 Some discussion topics/future concerns but looks good.

[benwtrent]

We need to make sure that in the future we remember to add these new tasks to our upgrade_mode settings. So that when users upgrade from 7.x -> 8.x they can do so without stopping these tasks themselves.

[dimitris-athanasiou]

I'll add a TODO.

[benwtrent]

When we have a FAILED state, adding a REASON field would be very helpful for debugging.

[dimitris-athanasiou]

There is no proper handling of failures yet. I'll make sure to add the reason in when I add failure handling.

[hendrikmuhs]

LGTM

It might be practical to break out the generic parts and PR them to master, so we can use them before the feature branch is merged

[dimitris-athanasiou]

@hendrikmuhs I think we need a usage in master before porting the abstract-get-action. I'll try to use it for get-filters soon.

[droberts195]

Since this is only going into a feature branch I'm happy to merge it now and resolve everything that's left in followups