Discover: Add yml setting to disable new fields API fetching

[flash1293]

In addition to the ui setting, there should be a yml setting to completely turn off fields API.

If this flag is set, the ui setting doesn't have any effect, and all code behaves as if the ui setting is set to true.

This is meant to make it possible to turn off the behavior for a lot of instances at once without messing with individual ui settings.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)