This commit removes some leniency from the plugin service which skips
hidden files in the plugins directory. We really want to ensure the
integrity of the plugin folder, so hasta la vista leniency.

* master:
  Discovery EC2: Remove region setting (elastic#23991)
  AWS Plugins: Remove signer type setting (elastic#23984)
  Settings: Disallow secure setting to exist in normal settings (elastic#23976)
  Add registration of new discovery settings
  Settings: Migrate ec2 discovery sensitive settings to elasticsearch keystore (elastic#23961)
  Fix throttled reindex_from_remote (elastic#23953)
  Add comment why we check for null fetch results during merge

This commit adds a test that hidden files are not skipped in the plugins
folder.