Cronjob intervals customization with CRONTAB_OFFSET

[cosyd]

• Currently, the CRONJOB_ITERATION parameter nicely allows to customize the intervals. However, every interval starts at :00, e.g. 12:00h, 13:00h, etc..

• The issue is that the Speedtest-Servers are typically overloaded at that time, resulting in distorted values - all Speedtests at :00 give way lower results than ones at uneven start times, like 12:13, 12:26, 12:39, whichever. I get way lower bandwith and pings at the :00 tests than any other time.

• This issue can be partially solved by putting an uneven CRONJOB_ITERATION parameter in, like CRONJOB_ITERATION=13. This gives good results at the times of 12:13, 12:26, etc. - but still one bad result at :00 start time.

• It would be nicer to be able to further customize the crontab entry with an additional parameter CRONTAB_OFFSET, applying an offset, see:
  https://stackoverflow.com/questions/12786410/run-cron-job-every-n-minutes-plus-offset
  This would give more valid test results, omitting the :00 start time altogether.