Better MySQL dependency detection in percona-toolkit

The percona-toolkit formula was stricly depending on mysql making it impossible to install when a user had maridb or percona-server installed instead of the standard mysql.

It now detects whatever mysql executable exists on the system before requiring the mysql dependency.
Does `percona-server` install a `mysql_config` file and is it in your `PATH`? Is it installed? If so it should be picked up and not try and install `mysql`.