Added Support Alertmanager

[nsano-rururu]

Description

This is a yelp / elastalert pull request with a few code changes and settings added to the content.

Yelp/elastalert#2228

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

alert:
  - "alertmanager"
alertmanager_hosts:
  - "http://alertmanager:9093"
alertmanager_api_version: "v2"
alertmanager_alertname: "Title☃"
alertmanager_annotations:
  severity: "error"
alertmanager_labels:
  source: "elastalert"
alertmanager_fields:
  msg: "message"
  log: "@log_name"
alert_subject: "Test {} 123 aa☃"
alert_subject_args:
  - "message"
  - "@log_name"
alert_text: "Test {}  123 bb☃"
alert_text_args:
  - "message"
filter:
  - query:
      query_string:
        query: "message:Quit AND @log_name:mysqld.general"
index: "mariadblog-*"
is_enabled: true
name: "a"
num_events: 2
realert:
  minutes: 5
terms_size: 50
timeframe:
  minutes: 5
timestamp_field: "@timestamp"
timestamp_type: "iso"
type: "frequency"
use_strftime_index: false