Stop pipeline without deleting it, when using centralized pipeline management

[lmtjalves]

With Centralized Pipeline Management enabled in Logstash, it would be nice to be possible to stop a pipeline, created in Kibana Pipeline Management, without having to delete it. So that we can later start it without having to create it again.

[bphenriques]

👍

[assaf79]

Have been working on a fix for this one.
aside from adding a switch in kibana to turn pipelines on and off,
there are a couple of ways to make this change visible to logstash itself:

1. a boolean indexed as a new field in the .logstash index. this requires changing the index in the elastic codebase, and possibly upgrading the existing index in previous versions of ES.
2. a boolean stored within the pipeline's settings. in this case logstash can either (a) filter using a script (as the field isn't indexed), or (b) filter returned pipelines in logstash's code.

#1 is cleaner, but I couldn't find any examples of how ES upgrades existing system indices during version upgrade.