Provide sentinel-proxy with device token

Modify sentinel-proxy's init script to generate ini file at /tmp/etc/sentinel/proxy.ini with all available config options including device_token. This ini file will be used by sentinel-proxy at it's startup.

To be implemented after #493 (closed)

Connected with turris/sentinel/proxy#2 (closed)

The device_token looks like proposed in turris/foris-controller/foris-controller-sentinel-module#1 (closed)

EDIT: The proxy config file was moved to /tmp/etc/sentinel-proxy.cfg
Currently, there's only one line:

device_token = <device_token>