WIP: reduce number of mails per day, from 15 down to 2, see #912591.

- master-bin/master_shell_runner: new script to run python scripts to
  redirect logs to htdocs/logs.
- conf/crontab-master.in: run detect_well_known_errors and piuparts-report
  through the new master_shell_runner script.

Signed-off-by: Holger Levsen <holger@layer-acht.org>