Correcting default image name
Correcting default image name

When using disk image builder deployments, the file name needs
to end with .qcow2.  Documented and changed the default file name.

juliakreger/bifrost,EntropyWorks/bifrost,openstack/bifrost,bcornec/bifrost,EntropyWorks/bifrost,devananda/bifrost,openstack/bifrost,bcornec/bifrost,juliakreger/bifrost,devananda/bifrost