[FEATURE] Use conf.d folder instead of main cnf file, if available on system

[MurzNN]

Is your feature request related to a problem?

When using mysql-formula to manage MySQL on fresh Debian/Ubuntu based linux systems, the formula stores custom changes into main /etc/mysql/my.cnf configuration file, instead of using /etc/mysql/conf.d folder to override needed settings only.

This give a problem when upgrading mysql package with deploying new default configuration file.

Describe the solution you'd like

Good solution for this problem can be don't touch the default /etc/mysql/my.cnf configuration file, and save SaltStack configuration into /etc/mysql/conf.d/saltstack.cnf file instead.

What do you think about this idea?