Inconsistency for accepted settings for ssl_verify_mode

[breml]

The documentation for ssl_verify_mode lists the following allowed values: none, peer, force_peer.

If an invalid configuration is provided, the following exception is raised (https://github.com/logstash-plugins/logstash-input-beats/blob/master/lib/logstash/inputs/beats.rb#L148): Using verify_modeset to PEER or FORCE_PEER, requires the configuration ofcertificate_authorities` where

1. the setting is called verify_mode instead of ssl_verify_mode
2. the allowed settings are written in all uppercase

In the code there are locations, where the case for this settings does not matter (https://github.com/logstash-plugins/logstash-input-beats/blob/master/lib/logstash/inputs/beats.rb#L180) and other locations, where case indeed does matter (https://github.com/logstash-plugins/logstash-input-beats/blob/master/lib/logstash/inputs/beats.rb#L224).

Stumbled over this, while debugging a similar situation as mentioned in #196 (PR: #182).