[openstreetmap/openstreetmap-website] Update embed.js.erb to use CDN test URLs (#2714)

[Grant]

Quick background for others not in the OSM Operations communications feedback loop: This is to test moving a small proportion of the `tile.openstreetmap.org` traffic to a 3rd party commercial CDN provider. We have been discussing it over the last few fortnightly OSM Ops meetings.

The point of the test is to evaluate:

1) How well the CDN performs?
2) What config we need to consider / set / create?
3) Gaining some metrics to help further decisions / evaluation?
4) Other observations? 

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