Debian: mysql_install_db is called on upgrade

A comment in debian/mariadb-server-10.1.postinst says: "can safely run on upgrades with existing databases". While this is true there're a few reasons not to do that:

• it increases installation time (it has to run rather heavy mysqld multiple times)
• as well as it increases mysqld downtime
• it may fail if database has some plugin specific configs (see MDEV-8437)
• there should be no need to run this script on upgrade: they should be handled by mysql_upgrade to
• RPM postin doesn't call it if database directory exists