Add support for setting 'migration.6_to_7.enabled'

[skrussell]

The title says it all really. I needed to be able to set the 'migration.6_to_7.enabled' config parameter, but couldn't find a way to do it with the existing module. I've added it as a simple Boolean option, defaulted to false, and it works for me.

[pcfens]

Thanks for the contribution. If I'm reading the docs correctly, that parameter only has to be set on one machine so the schema is updated in ES. If that's the case then I'm not sure it'd be something that we'd want to apply to every machine (just 1 to update the schema?). We also don't manage the initial schema setup, so I don't know if we want to manage upgrades but not initial setup.

We don't ship directly to ES, so I may be misunderstanding this entirely too.

[skrussell]

If I'm completely honest, I am not too sure what this setting actually does. I am simply re-implementing a puppetised version (for eventual production usage) of an already working development environment. Removing the setting causes the following error(s) on our logstash server: Detected a 6.x and above cluster: the 'type' event field won't be used to determine the document _type {:es_version=>7}, so for us it is definitely required.

Regardless though, it is definitely a valid setting within the filebeat.yml file, but until I made these changes, I could not find a way to set it using this module. If you're not overly keen on the method of implementation (i.e. the boolean flag), perhaps a more generic way of adding any additional top level setting would be preferred?