Make MAP-URL configurable (for custom geo-tile server)

I think that change url dynamic for installations with offline openstreemap since web, without change code of plugin would be great. It is useful for business environment.

## Possible Solution
Set DEFAULT_URL map as parameter in plugin configuration

`https://github.com/Graylog2/graylog2-server/blob/3dfae8271be95767ff29b23d64737cf7d012116b/graylog2-web-interface/src/components/maps/widgets/MapVisualization.jsx#L29`

* Graylog Version: Any version
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