Adding optional environment variables for setting mysqld and mysqld_inst...

I'm using CentOS 6.2 and running Test::mysqld causes the script to hang forever because of the scripts that ship with the mysql-server package.

This change allows me to set environment variables that will point to the proper places so I don't run into this problem in my or other peoples tests.

I'd also like to write some code that could read these options from a config file if it exists but though this would be a good first step. If you're interested in willing to me adding a config file option please let me know.

Thanks