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.
I added the :recommended option, but when I saw the output generated:

````shell
$ brew install percona-toolkit        
==> Installing percona-toolkit dependency: mysql
Error: Cannot install mysql because conflicting formulae are installed.

  percona-server: because mysql, mariadb, and percona install the same binaries.

Please `brew unlink percona-server` before continuing.

Unlinking removes a formula's symlinks from /usr/local. You can
link the formula again after the install finishes. You can --force this
install, but the build may fail or cause obscure side-effects in the
resulting software.
````

I decided to implement an additional check because there isn't a hint in the output for the `--without-mysql` option needed to skip the dependency.

Is there a way to display a suggestion to the user in this case?