Provide "my-default.cnf" to "mysql_install_db" from a different location.

At the moment the "my-default.cnf" file gets overwritten in the "support-files" folder of the provided MySQL distribution. This can be inconvenient in an environment where a user (or build system) does not have write access to the folder of the installed MySQL.

To get over this problem I used to copy the entire MySQL distribution into target. This is a time consuming and unnecessary step as the "mysql_install_db" allows to take a "my-default.cnf" from a different location using the "--defaults-file" parameter.