Simplify setting API key between mapzen.js and Tangram Play

[rmglennon]

This came in through support (see Desk 1134).

The user was trying to do this in Tangram Play at the top of the yaml file

global:
        sdk_mapzen_api_key: 'mapzen-api-key-here'

and not realizing that url_params was necessary under sources. The user kept getting a warning about missing an API key.

Discussion from support chat: "sdk_mapzen_api_key works when you’re using mapzen.js but Play never learned that trick"