report about disk usage says list is empty

Issue

• There exists an openshift 3.1 installation (2nodes + 1 master/node co-located):
  • On the master df reports 95% disk in use, but when the CloudForms report VM with free space<20% runs, it gives an empty list, as no VM have less than 20% disk free
  • all the nodes are VMs
• The problem is the same if the reports are run manually