master: lazy create PNG in cron

As Christoph Biedl reported :

    In cron mode, munin-graph not only creates graphs for all services but even
    does not take into acount that the bigger scale graphs (week, month, year)
    may be skipped for some time.

    This puts even more load onto the system than cron graphing does anyway. Is
    there a specific reason for that? Especially since the code for freshness
    check is already partially there?

Closes: #1394
Merge branch 'fix-1394' into stable-2.0