Implement plugin config prefix

[w33ble]

Closes #6304

Allows plugins to optionally prefix their config values via a new configPrefix parameter

When configPrefix is set, it uses that value for the config values. Otherwise it uses the default, which is simply the plugin id value.

Both of these are valid:

// without configPrefix
return new kibana.Plugin({
  id: 'myPlugin',
  config: function (Joi) {
    return Joi.object({
      mySetting: Joi.boolean().default(true),
    });
  },
  init: function (server, options) {
    const config = server.config();
    config.get('myPlugin.mySetting'); // returns true
  }
});

// with custom configPrefix
return new kibana.Plugin({
  id: 'myPlugin',
  configPrefix: 'customPluginPrefix',
  config: function (Joi) {
    return Joi.object({
      mySetting: Joi.boolean().default(true),
    });
  },
  init: function (server, options) {
    const config = server.config();
    config.get('customPluginPrefix.mySetting'); // returns true
  }
});

[spalger]

path isn't used here

[spalger]

LGTM

[spalger]

passing objKeys here is unnecessary. Joi is itterating the own properties of objKeys and adding them as keys, but since it's an array it has no own properties and does nothing.

[w33ble]

Yup, I read the Joi docs wrong before.

The call to .keys() is required, otherwise the validation fails (and the tests do as a result), but it just needs to be given an empty object, since the actual keys are appended after the fact.

[w33ble]

There appears to be what looks like a race condition here, where the keys in the schema are changing and then the validation fails.

I've used this to prefix the XPack settings with xpack, and I see the keys in getSchema/convertToSchema changing.

[ 'graph', 'monitoring', 'reporting' ]
...later
[ 'graph', 'monitoring' ]
...later still
[ 'graph', 'monitoring', 'shield' ]

And then Kibana fails due to schema issues: ValidationError: child "xpack" fails because ["reporting" is not allowed]

[w33ble]

Issue was related to disabled plugins. Should be fixed now. @spalger I broke out the unset module and added tests, let me know if there's a scenario I missed in there though.

[w33ble]

jenkins, test it

[spalger]

jenkins, test it