Sentinel:Proxy Certificate renewal

The main goal of this merge request is to implement automated certificate renewal for Sentinel:Proxy via Sentinel:Certgen cron job (second task from #383 (closed)). When a certificate is issued (or renewed), the Proxy should be restarted which is handled with implemented hooks directory in Certgen (turris/sentinel/certgen!12 (merged)).

I have converted Certgen to regular python package (turris/sentinel/certgen!10 (merged)) and so I have changed its Makefile. I have dropped SECTION, CATEGORY and SUBMENU as I don't see these parameters in other python packages neither. If it is required, or a good practice to have them there, I should add them to other packages I have managed as well.

Please review these changes.