Distributed/Geographical Maps do not show images anymore

The Distirbuted and Geographical Map in OpenNMS use a so called tile server to generate a graphical representation from the data open street maps provides.

OpenNMS uses the tileserver from MapQuest as a default.
However, as of 2017-07-11, the tile servers cannot be used anymore.
See http://devblog.mapquest.com/2016/06/15/modernization-of-mapquest-results-in-changes-to-open-tile-access/ for more details about the reasons.
The attached screenshot shows the result of still accessing the discontinued API.

To address this issue the following problems must be solved:

• find another default tile server provider
• use the gwt.openlayers.url property in the node-map-application (Geographical Maps) as well, as it is currently hard coded
• Allow setting additional "TileLayer" properties, in order to set subdomains, attributions, etc. See http://leafletjs.com/reference.html#tilelayer-options. Especially the attribution option is crucial as if we are going to fallback to use http://tile.openstreetmap.org/ as the default. Their usage policy requires to set the attribution correctly. At the moment we cannot set it at all
• Maybe host our own tile server

Suggested default tile server: http://tile.openstreetmap.org/, but may result in problems described here: http://wiki.openstreetmap.org/wiki/Tile_usage_policy

List of available tile servers: https://leaflet-extras.github.io/leaflet-providers/preview/

Workaround for Distributed Maps:
Set the property gwt.openlayers.url in the opennms.properties to another tile server:

gwt.openlayers.url=http://tile.openstreetmap.org/${z}/${x}/${y}.png