enabled systemd unit does not start at boot

The unit file was modified:

Before:

[Install]
RequiredBy=docker.service

After:

[Install]
WantedBy=multi-user.target

After that modification I did not reenable the Unit. Turned out that this is needed for systemd to reconfigure:

$ sudo systemctl reenable flanneld
rm '/etc/systemd/system/docker.service.requires/flanneld.service'
ln -s '/usr/lib/systemd/system/flanneld.service' '/etc/systemd/system/multi-user.target.wants/flanneld.service'

I find no evidence in your configuration that this service should be started at boot time. systemd allows two ways of enabling a service so it is started at boot time:

chkconfig flanneld on

Or:

systemctl enable flanneld

In fact, the first is a legacy way of calling the second one, and I'm not sure if the chkconfig package is installed by default, but you can install it using apt-get or yum.