puppet-compiler: alleviate disk space problems

In order to solve the long-standing disk space problems in the
puppet-compiler ci job, we do the following:

- mount in bind the space-eating dirs from /mnt, where we have a large
  secondary disk.
- set up a weekly cronjob that should clean up leftovers

Change-Id: Iee31f83e958a856c46c7318cefa70781d8a5d2bb
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>