When the multi-instance systemd service file was chosen it effectively
relied on /etc/my.cnf.d/my{instancename}.cnf file to define its
configuration file. This is problematic if running along side a
single instance mariadb service which has /etc/my.cnf that reads all
configuration file /etc/my.cnf.d/*.cnf.

To prevent the service from auto starting up if a user has this
previous configuration ConditionPathExists=!@sysconf2dir@/my%I.cnf
to ensure that a user with the previous configuration isn't
started in a non-intended mode. Documentation in the service file
(should be release notes too), described a recommended migration.

A new approach was to use --defaults-group-suffix=.%I as an
arguement to mysqld and let the user define a [mysqld.{instancename}]
group within the configuration file. This way existing global
mysqld configuration options are read with the instance name
having special overrides of datadir, port, socket etc.

A systemd environment variable MYSQLD_MULTI_INSTANCE is used in the
defination as it give the user flexability to use multiple
segregation mechanisms between services. This is used multiple
times within the service which all needed to be kept consistent.

Another notable change is mysql_install_db being part of the
ExecStartPre. This provides and auto-initialization for users
that run multiple instances.