ES7 Initial Bootstrap Script - use at your own risk - use tags and ec2 discovery. install boto3 somewhere, ensure you have the cluster name set in the config and ec2 discovery configured.

gistfile1.txt

#!/usr/bin/env python

# It's up to the enduser to make a backup of their config incase things break.  If you're bootstrapping, this should only run once.
# You'll also need to install boto3 somehow.  This should be reasonable python version agnostic.
import boto3
import requests
import yaml
import json

# src and dst file
esfile = '/etc/elasticsearch/elasticsearch.yml'
# Read our current yaml
esyml = yaml.load(open(esfile))
hosts = list()

# Get our region from metadata because scalability and laziness.
r = requests.get(
    "http://169.254.169.254/latest/dynamic/instance-identity/document")
response_json = r.json()
region = response_json.get('region')

# Get the instances filtering on master and our cluster name.
client = boto3.client('ec2', region_name=region)
response = client.describe_instances(
    Filters=[
        {'Name': 'instance-state-name',
         'Values': [
                    'running'
        ]
        },
        {'Name': 'tag:ES_CLUSTER',
         'Values': [
             esyml['cluster.name']
         ]
         },
        {'Name': 'tag:ES_MASTER',
         'Values': [
             'true'
         ]
         }
    ])


# Get node names. (because filters, right?)
for i in response['Reservations']:
    for j in i['Instances']:
        for k in j['Tags']:
            if k.get('Key') == 'ES_NODE':
                hosts.append(k.get('Value'))


hosts.sort()

# Ensure previous values are nuked
try:
  del esyml['cluster.initial_master_nodes']
except:
  pass

esyml['cluster.initial_master_nodes'] = hosts



# Make your own backups
with open(esfile, 'w') as outfile:
    yaml.dump(esyml, outfile, default_flow_style=False)

See https://discuss.elastic.co/t/elasticsearch-7-0-bootstrapping-in-aws/176491/8 for related discussion.