add env variables for new ttn-integration

[noerw]

when sensebox/ttn-osem-integration#13 is merged, new env variables are supported:

• TTN_OSEM_ttn_app
• TTN_OSEM_ttn_key
• TTN_OSEM_authtoken: commaseparated list of auth keys that are allowed to request authenticated routes when sent in the 'authorization' header