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.
It looks like this is due to a0a93f1b3b7b2be9b8a319be91086ffe220f8e32