Require Device token to run
Sentinel Proxy should require and send Device token as well as device serial number.
Foris is responsible to generate such token (turris/foris#158). Please discuss with @shenek required format (I suggest something like 32-bytes hex/base64 string).
This token is potentially confidential, so it would be better to pass it via a file, stdin or an environment variable in init script.
device_token shall be read from
/tmp/etc/sentinel/proxy.ini generated by init script. We will preserve the option to pass it via cli argument for debug purposes. See /turris/turris-os-packages#518