Default VM image is set also for second KVM disk

[YanChii]

Steps to reproduce:
In datacenter settings set VMS_DISK_IMAGE_DEFAULT to any image. Then try to add second disk to any VM.
Log file (/opt/erigones/log/main.log in mgmt01.local):

...
with response (400): {'status': 'FAILURE', 'result': {'image': [u'Cannot set image on disks other than first disk.']}, 'task_id': '1d1d1-6f75849b-34fc-4027-a15b'}

It should not take the default image value for disks other than the first one.