Fix debian image download URL for QEMU
https://lab.nexedi.com/nexedi/slapos/blob/master/software/kvm/test/test.py fails:
slapgrid_sr: 2018-12-18 14:29:39 slapos[1645] INFO Installing debian-amd64-netinst.iso.
slapgrid_sr: 2018-12-18 14:29:39 slapos[1645] INFO Downloading http://cdimage.debian.org/cdimage/release/current/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso
slapgrid_sr: 2018-12-18 14:29:39 slapos[1645] INFO networkcache: Downloading slapos-buildout-55f6b9cefeb8aa718ae00a12152d956d...
slapgrid_sr: 2018-12-18 14:29:39 slapos[1645] INFO networkcache: ignored unhandled exception at /srv/slapgrid/slappart9/srv/testnode/byn/inst/test0-0/tmp/soft/2f1311df6c39f8847735be55a305d0c4/eggs/slapos.libnetworkcache-0.16-py2.7.egg/slapos/networkcachehelper.py:104
slapgrid_sr: 2018-12-18 14:29:39 slapos[1645] INFO BadStatusLine: ''
slapgrid_sr: 2018-12-18 14:29:39 slapos[1645] INFO Cannot download http://cdimage.debian.org/cdimage/release/current/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso from network cache.
slapgrid_sr: 2018-12-18 14:29:40 slapos[1645] INFO While:
slapgrid_sr: 2018-12-18 14:29:40 slapos[1645] INFO Installing debian-amd64-netinst.iso.
slapgrid_sr: 2018-12-18 14:29:40 slapos[1645] INFO Error: Error downloading extends for URL http://cdimage.debian.org/cdimage/release/current/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso: HTTP Error 404: Not Found
slapgrid_sr: 2018-12-18 14:29:40 slapos[1645] ERROR Failed to run buildout profile in directory '/srv/slapgrid/slappart9/srv/testnode/byn/inst/test0-0/tmp/soft/2f1311df6c39f8847735be55a305d0c4'
slapgrid_sr: 2018-12-18 14:29:40 slapos[1645] INFO Finished software releases.