Allow koan to install "image" based virtual machines

Currently, koan reproducibly breaks when trying to access a virtual
machine definition, if the virtual machine is inheriting an "image":

$ koan --display --system=system1
DEBUG: get_data(<koan.app.Koan instance at 0x7f613ea0bd40>,system,system1)
...
Fault: <Fault 1: "<type 'exceptions.AttributeError'>:'NoneType' object has no attribute 'arch'">

And in cobbler.log:

  File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 1970, in _dispatch
    return method_handle(*params)
   File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 1481, in get_system_for_koan
    arch = distro.arch
Merge pull request #508 from chutzimir/koan-image

Allow koan to install "image" based virtual machines