Since Andrej ran in similar issue today, I'm posting for others:
When installing oVirt 4.2 first alpha release from scratch on fresh Centos
7 minimal, adding host failed. Unfortunately, I don't have log messages
anymore, but I remember last errors were misleadingly related to ovn (no
matter the ovn was not configured on the engine).
The issue was resolved by manual installation of python-netaddr package on
the host. Subsequently, adding the host in webadmin passed.
What package should require the python-netaddr? ovirt-provider-ovn-driver?
I hope it helps,