Excuse me for being a bit off-topic by hijacking the slightly different issue here to provide solution to similar case. Since I stumbled over this thread by trying to solve my somewhat related issue I beg for your pardon.
I was having issues with starting mysql in an lxc container running Ubuntu 16.04. I tried
update-rc.d mysql defaults as well as
systemctl enable mysql.service several times but mysql didn't come up on boot. There wasn't even a log entry regarding some error to fix.
The lxc container is getting its network configuration via host's lxc configuration file but still was configured to start DHCP client enabled by default. By disabling DHCP in lxc container I stumbled over solution by accident. This was the reason for mysql not starting as it was waiting for DHCP client to finish. And DHCP didn't finish due to missing any response from unavailable DHCP service.
So, just in case you stumble over the same question here and find yourself in a similar dilemma try this: Edit
/etc/network/interfaces and comment-out lines reading
iface eth0 inet dhcp