Logstash config path need to check the permission of the file.

The `config : path` option only check if the file exists, we don't check if we have the right permission to read the file, in circumstances this will lead to confusing error.

moved from https://github.com/logstash-plugins/logstash-input-beats/issues/116

```
An unexpected error occurred! {:error=>#, :backtrace=>["/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-input-beats-3.1.0.beta4-java/lib/logstash/inputs/beats.rb:173:in stop'", "/usr/share/logstash/logstash-core/lib/logstash/inputs/base.rb:88:indo_stop'", "org/jruby/RubyArray.java:1613:in each'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:366:inshutdown'", "/usr/share/logstash/logstash-core/lib/logstash/agent.rb:252:in stop_pipeline'", "/usr/share/logstash/logstash-core/lib/logstash/agent.rb:261:inshutdown_pipelines'", "org/jruby/RubyHash.java:1342:in each'", "/usr/share/logstash/logstash-core/lib/logstash/agent.rb:261:inshutdown_pipelines'", "/usr/share/logstash/logstash-core/lib/logstash/agent.rb:123:in shutdown'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:237:inexecute'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67:in run'", "/usr/share/logstash/logstash-core/lib/logstash/runner.rb:157:inrun'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132:in run'", "/usr/share/logstash/lib/bootstrap/environment.rb:66:in(root)'"], :level=>:fatal}
```