Require settings `locale` property

[drewdaemon]

With #2164 we got the ability to pass a locale to our charts. This was accomplished in Kibana with elastic/kibana#168571.

However, locale is currently an optional property. This increases the risk that it will be overlooked when new charts invocations are added to Kibana, resulting in subtle localization bugs.

We should consider requiring locale to make sure it is always provided.

[nickofthyme]

Good point, it looks like the locale prop is optional on the Settings spec but even if it was required there'd be no way to require a Settings spec, although most charts in kibana use this.

I don't think there is a current solution to fix this given how elasstic charts is built but I think the work in #1424, to use a charts provider, would be useful for exactly these types of cases where we want global chart defaults.