Enhancements: Manage ILM policies in the same way as managing ES templates

[prehor]

#805 introduces great ILM support. The only thing I miss is the ILM policy management right in the Elasticsearch output plugin:

output {
  elasticsearch {
    ...
    manage_policy => true
    policy => "path/to/policy.json"
    policy_name => "my_policy"
    policy_overwrite => true
  }
}