Add log rotation

[Haw Loeung]

Hi,

| app-path: /srv/discourse/current

| ubuntu@juju-f9feab-prod-community-ubuntu-com-8:/srv/discourse/current/log$ ls -la
| total 9516356
| drwxr-xr-x 2 www-data www-data 4096 Dec 11 2018 .
| drwxr-xr-x 21 www-data www-data 4096 Oct 21 2017 ..
| -rw-r--r-- 1 www-data www-data 0 Oct 21 2017 .gitkeep
| -rw-r--r-- 1 www-data www-data 9521394490 Aug 8 03:24 production.log
| -rw-r--r-- 1 www-data www-data 223244171 Aug 8 03:21 unicorn.stderr.log
| -rw-r--r-- 1 www-data www-data 85986 Oct 23 2017 unicorn.stdout.log

The charm should ship out a logrotate config to rotate these logs.

[Junien F (axino)]

Related :

https://yhbt.net/unicorn/SIGNALS.html (credits @jk0ne) : "USR1 - reopen all logs owned by the master and all workers See Unicorn::Util.reopen_logs for what is considered a log."

https://github.com/discourse/discourse/blob/master/config/logrotate.conf (not super helpful...)

Worst case, we could use the "copytruncate" logrotate stanza.