slapos/collect: enable disk usage

• 'slapos node collect' runs once a minute as indicated in cron.d file.
• 'slapos node collect' now writes in the slapos-node-collect.log file.
• All the tables of collector.db can be filled automatically.
• The command line 'du -s' runs once very day (by default). This can be changed in slapos.cfg by adding a collect section.
• I added two tests in test_collect.py