Variable extrapolation in configuration doesn't work in sub config.

[ph]

orignally reported in discuss at: https://discuss.elastic.co/t/environment-variables-not-replaced-in-external-configuration-file/124690

• Version: 6.3.0 and 6.2.2
• Operating System:
• Steps to Reproduce:

Make theses changes to you YAML configuration

# filebeat.yml
custom.testing: ${CUSTOM_TESTING_ENV}
 filebeat.config.prospectors:
      enabled: true
     path: test.yml

# test.yml
- type: log
  enabled: true
  paths:
    - "/tmp/${custom.testing}"

Results into: Exiting: missing field accessing '0.paths.0'

Same problem if you using an environment variable directly in the sub configuration it won't get replaced.

[ph]

When I've added the keystore #5737 in libbeat I've made some changes to the config handling, I will need to check if it got introduced by it or not.