jenkins: logrotate main file on a daily basis

The Jenkins main log is rotated on a weekly basis. Our setup produce a
lot of logs, so it seems better to rotate on a daily basis (keeping 20
days of archives instead of 52 weeks).

The current log file for four days is 3.1 GBytes.

Also adds ensure => present to /etc/default/jenkins

Change-Id: Id6134525872f292056fd02c7fe290345d2223c14