Make default_watch configurable

[gmoskovicz]

Describe the feature:

The default watch is configured in a static file here: https://github.com/elastic/kibana/blob/fe4609647dd2a7a7fedfb23d63f5886a24eacbe1/x-pack/plugins/watcher/public/models/watch/default_watch.json.

I would like to request this to be configurable in some sort of way, in order to have a different template.

Describe a specific use case for the feature:

This can help when you have 500 users and every time users create a watch they follow a common template that can be pulled from a file/centralized config. The default template that we ship by default doesn't really match the needs, but there is a common pattern for all watches in our side.