Adhere to the Debian idioms more closely for some config values.

I'm not even sure this role works properly on Debian/Ubuntu currently, because the init scripts look for `/etc/mysql/my.cnf` but the role actually writes to `/etc/my.cnf`.  This should address that, among with some other minor things.

`/etc/mysql/my.cnf` by default does an "include" of `/etc/mysql/conf.d/*.cnf` so rather than writing the main config file, I changed the Debian version of the role to write to the `conf.d/` directory.

Finally, I changed the default `mysql_socket` value based on the distro.