buildbot master: clean logs

[sascha_silbe]

We need to cleans logs periodically, see  upstream instructions.

[sascha_silbe]

I've (manually) cleaned the server logs (twistd.log.*) for a start (about 2GB!), but we still need to set up a cron job to do periodical cleaning.
Not sure for how long to keep the build logs, there are some Trac tickets referencing old build logs open for ages. More recent logs show the same error, but the link would break if we removed the old logs.

[sascha_silbe]

I've set up a cron job to remove twistd.logs except for the first ten rotations.

Build logs are  automatically compressed by default since 0.7.10.

We should scan the tickets for old referenced build logs and attach them to the ticket so we can prune the old logs from buildbot, but that's a task for some other day when the build logs (rather than just twistd.log) get out of bounds.