Comment 1 for bug 1580766

Looks like this is caused by a conflict with the lxc-net service.

May 11 21:01:00 host2-osa dnsmasq[1100]: failed to create listening socket for 10.100.100.1: Cannot assign requested address
May 11 21:01:00 host2-osa dnsmasq[1100]: FAILED to start up
May 11 21:01:00 host2-osa lxc-net[994]: Failed to setup lxc-net.

In 14.04 the service was disabled, it doesn't seem to be disabled in systemd systems.

The options would be to either disable lxc-net, or to remove /etc/network/interfaces.d/lxc-net-bridge.cfg and let lxc-net handle managing the bridge.