pluto-database-settings install fails on Xenial because root@127.0.0.1 mysql user does not exist.
pluto-database-settings fails to install, because root@127.0.0.1 mysql user does not exist, therefore the GRANT ALL PERMISSIONS command to root@127.0.0.1 will subsequently fail.
solution is to put a CREATE USER root@127.0.0.1; before the GRANT ALL PERMISSIONS call.