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,