systemd: failed to mount NFS share: mount.nfs: Network is unreachable until later in boot process

On CentOS 7 network filesystem mounts depend on network-online.target.
The network-online.target is reached once an interface is up and an ip has been set.
This presumes the first ip on the first interface to get up is enough for the hostname to be resoved. This is however doesn't have to be the case.

We can write systemd services to test if network is supposed to be flagged as "up" and make the network-online.target dependent on that service.

For a template look at: https://gitlab.com/ggeurts/extend-network-online.target