The process of testing upstream configuration changes involves "cloning"
existing services. These clones are given their own distinct service key
which is then sent as the IFTTT-Channel-Key header when running tests or
live requests through the service. By allowing for multiple keys to be
used in parallel we should make it slightly easier to propose, test, and
deploy backwards compatible configuration changes upstream. One example
use case for doing that would be adding a new trigger to the
"production" deployment.

* Rename CHANNEL_KEY to CHANNEL_KEYS
* Make CHANNEL_KEYS an array in default.cfg
* Check IFTTT-Channel-Key header vs entire list of allowed keys