Cherry-pick #24480 to 7.x: Improve ILM policy and alias setup log output

[urso]

Cherry-pick of PR #24480 to 7.x branch. Original message:

• Bug

What does this PR do?

The idxmgmt has not all 'information' to properly log the outcome of the
policy and status setup. The PR moves the reporting to the ILM package
directly, and 'unifies' the logic of EnsurePolicy and EnsureAlias.

Why is it important?

The change fixes the status reporting for the ILM and index alias setup, that is written to the logs.
With the fix one can tell if some resources have been installed, or not, depending on the actual settings in the configuration file. Before it was potentially reported that a policy/alias has been created, although the creation of the resources was disabled.

The change also updates EnsurePolicy/EnsureAlias to follow a similar pattern, which makes it a little easier to follow the code. We now allow EnsureAlias to create the first index and alias if check_exists: false and overwrite: true. If the alias already exists, no new alias will be created, as Elasticsearch will return an error.
This change in semantics fixes a setup that has ILM enabled with aforementioned settings, potentially indexing without the appropriate alias/index being setup ahead of time.

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes ILM setup misleads user when check_exists is false #24046

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Pinging @elastic/integrations-services (Team:Services)

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Build stats

• Build Cause: Pull request #24794 updated

• Start Time: 2021-03-26T16:06:05.160+0000

• Duration: 110 min 11 sec

• Commit: 4866f85

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Test	Results
Failed	0
Passed	46297
Skipped	5146
Total	51443

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Test	Results
Failed	0
Passed	46297
Skipped	5146
Total	51443