Add new alerter for a more flexible HTTP Post

[lepouletsuisse]

Description

Add a new alerter HTTP Post 2 which is a more flexible version of the HTTP Post alerter.
This alerter remove the difference between http_post_payload and http_post_payload_static to privileged only one field http_post_payload which allow to use Jinja2 templating to inject fields in a HTTP Post body.
I kept the old behavior of http_post_payload in http_post_raw_fields in case you want to keep the type of a field (a number for example).
The Jinja templating works for the keys and also the values, providing total flexibility for the users to send HTTP request with any format using the fields of the matches.

This PR resolve (at least partially):

• Add alert_text support for HTTP POST #165

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

• I created a new alerter so it doesn't break the old alerter HTTP Post and doesn't require a major version change.

[nsano-rururu]

I don't seem to fix loaders.py

[nsano-rururu]

How to write changelog is different from other people. Please correct.

[nsano-rururu]

Does this really work?

[nsano-rururu]

It looks like HTTPPost2Alerter isn't called from anywhere. If you specify alert: post, it will be called HTTP Post Alerter as it is.

[nsano-rururu]

If you want to keep the HTTPPostAlerter, you may use the old one and the new one at the same time. If so, shouldn't the setting name be http_post2_ setting name at the beginning so that it doesn't collide?

[lepouletsuisse]

I don't seem to fix loaders.py

As I said, it partially fixed the issue. My purpose is not to fix the loader, only to create a more flexible HTTP alerter.

It looks like HTTPPost2Alerter isn't called from anywhere. If you specify alert: post, it will be called HTTP Post Alerter as it is.

Yeah, my bad. I'm not really familiar with this code, so I forgot to add the post2 to the loaders.py. It's fixed now.

If you want to keep the HTTPPostAlerter, you may use the old one and the new one at the same time. If so, shouldn't the setting name be http_post2_ setting name at the beginning so that it doesn't collide?

Yes totally, thanks for pointing it out! I'm fixing this in a new commit

[nsano-rururu]

I think you should add the validation check setting to schema.yaml. There is a description of HTTP POST settings in 440 to 445. When you run the rule, jsonschema can check the settings in schema.yaml and check the data type if there is a key, and if they are different, you can raise an exception to prevent the rule from running.
https://github.com/jertel/elastalert2/blob/master/elastalert/schema.yaml#L440

[nsano-rururu]

@jertel

Is there anything else you care about?