Add option to easily disable outputs

[tsg]

One currently needs to comment out the whole section to disable an output. Because metricbeat has the option to use enable: false to quickly disable modules, it makes sense to add the same for the outputs at libbeat level.

[tsg]

Loosely related to #1417.

[drenalin23]

Any idea why this was removed from filebeat before 1.0.0 version? It made it into the filebeat cookbook (which now doesn't work as expected by reading the docs) and would be an extremely useful option when working configuration management to have a toggle for nodes/roles/environments instead of having to add/remove/comment out entire sections. Sorry if I'm ignorant of the overall architecture but hoping that adding this back to libbeat might push it upstream to filebeat.

In my use case I was rolling out filebeat to several environments, so wanted to setup the same config everywhere and then rollout it one environment at a time, planning on setting enabled to true as I went along.

[drenalin23]

For reference if this is implemented it is useful in the chef supermarket cookbook. vkhatri/chef-filebeat#55

[andrewkroh]

Original proposal to remove: https://github.com/elastic/libbeat/issues/264

Summary: We had both enabled and disabled options with varying defaults and it was confusing. Adding back just enabled should be fine if the default is consistent everywhere that it is used.

[tsg]

This should be done now in master.