Ubuntu upgrade from 14.04 LTS to 16.04 failed - mysql-server-5.7 dependecies fail

Have you checked: https://bugs.launchpad.net/ubuntu/+source/mysql-5.7/+bug/1571865 (linked from the XenialXerus release notes at https://wiki.ubuntu.com/XenialXerus/ReleaseNotes#MySQL_5.7)?

It explains that due to changes in the name of some configuration directives, starting mysql fails. It also mentions three solutions, the first of which solved my problem.