Elasticsearch version (bin/elasticsearch --version): 6.0.0-rc2

Plugins installed: [discovery-file, found-elasticsearch, repository-s3, x-pack]

JVM version (java -version): 1.8.0_144

OS version (uname -a if on a Unix-like system): Alpine Linux

Description of the problem including expected versus actual behavior:
The discovery-file plugin always emits the deprecation warning regarding the discovery.type config setting regardless of whether or no it is used.:

[2017-09-15T12:19:59,562][WARN ][org.elasticsearch.deprecation.discovery.file.FileBasedDiscoveryPlugin] Using discovery.type setting to set hosts provider is deprecated. Set "discovery.zen.hosts_provider: file" instead

Steps to reproduce:

1. install discovery-file plugin
2. add discovery.zen.hosts_provider: file to ES config
3. ensure discovery.type is missing from ES config
4. start ES