Add default value for pidfile

The following warning is issued during fail2ban log rotation (and also during fail2ban start/stop):
```
/etc/cron.daily/logrotate:
WARNING 'pidfile' not defined in 'Definition'. Using default one: '/var/run/fail2ban/fail2ban.pid'
```
... due to this line in /etc/logrotate.d/fail2ban:
```
fail2ban-client set logtarget /var/log/fail2ban.log >/dev/null
```

In order to suppress this warning, this pull request simply adds a default pidfile entry in /etc/fail2ban/fail2ban.conf.