treecompose: Add ability to automatically detect atomic host IP

When creating imagefactory images, we now detect if there is a "default"
KVM network and if so, we grab its IP and dynamically substitute it into
the KS file.  This prevents a difficult to debug error for users who
use a different IP/subnet scheme on their KVM networks.

If we cannot detect the network and IP, we suggest the user static define
it in the KS file.

packaging/rpm-ostree-toolbox.spec.in: Added libvirt-python as a runtime
dependancy.
Merge remote-tracking branch 'projectatomic/master' into libvirt
imagefactory: Fix bug where if default network not found

In my initial patch, even if the used hard-coded, as they
were advised, the OSTREE_HOST_IP in the kickstart it would
not proceed due to the fatal error while checking network
names.  This patch properly corrects that.