Reloading the config breaks legacy config validation

[joshdover]

Reloading the config breaks legacy config validation when a SIGHUP is received.

Steps to reproduce:

• Set a valid config for an xpack plugin, such as xpack.licensing.api_polling_frequency: 30000
• Start kibana
• Send a SIGHUP signal to Kibana
• Validation error in logs:

  { ValidationError: child "xpack" fails because ["licensing" is not allowed]
  at Object.exports.process (/kibana/node_modules/joi/lib/errors.js:196:19)
  at internals.Object._validateWithOptions (/kibana/node_modules/joi/lib/types/any/index.js:675:31)
  at module.exports.internals.Any.root.validate (/kibana/node_modules/joi/lib/index.js:146:23)
  at Config._commit (/kibana/src/legacy/server/config/config.js:132:34)
  at Config.set (/kibana/src/legacy/server/config/config.js:103:10)
  at Config.extendSchema (/kibana/src/legacy/server/config/config.js:75:10)
  at _lodash.default.each.child (/kibana/src/legacy/server/config/config.js:64:14)
  at arrayEach (/kibana/node_modules/lodash/index.js:1315:13)
  at Function.<anonymous> (/kibana/node_modules/lodash/index.js:3393:13)
  at Config.extendSchema (/kibana/src/legacy/server/config/config.js:63:30)
  at new Config (/kibana/src/legacy/server/config/config.js:54:10)
  at KbnServer.applyLoggingConfiguration (/kibana/src/legacy/server/kbn_server.js:197:20)
  at TapSubscriber.update$.pipe [as _tapNext] (/kibana/src/core/server/legacy/legacy_service.js:85:24)
  at TapSubscriber._next (/kibana/node_modules/rxjs/internal/operators/tap.js:59:27)
  at TapSubscriber.Subscriber.next (/kibana/node_modules/rxjs/internal/Subscriber.js:66:18)
  at CombineLatestSubscriber.notifyNext (/kibana/node_modules/rxjs/internal/observable/combineLatest.js:97:34) name: 'ValidationError' }
  

There a couple problems here:

1. This ValidationError should be triggered when Kibana first starts up, not only config reload.
2. Config paths should not be allowed to overlap

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[mshustov]

@joshdover it seems we stumbled upon this bug again #43513 (comment)