Add parameter for mapbox api key to demos containing superset #204

We used to roll out a mapbox api key with our Superset demos, but removed it due to the terms and conditions of mapbox.

Currently demos therefore don't display maps, until the user manually replaces the api key in a secret.

We should pull out the content of this secret as a parameter, so that users can specify an api key during start of the demo.

Also document this change in all relevant demos and mention that an api key needs to be obtained and provided if maps are to be displayed.