Include support for using custom path to cacerts

[woodtechie1428]

Enhancement request to include support for adding a custom path to the cacerts used by the plugin. This would be helpful in scenarios where the logstash server's access to the internet is via proxy that does SSL inspection.

For reference, this can be similar to how elasticsearch output plugin achieves the same functionality.

output {
  elasticsearch {
    ...
    ssl => true
    cacert => '/path/to/cert.pem' 
  }
}