modify pt-toolkit to use --login-path and .mylogin.cfg

MySQL 5.6 allows encrypted passwords to be stored in .mylogin.cfg files, created by mysql-config-editor. To allow scripting and automation of tasks using pt-toolkit tools, it would be very helpful if the tools were modified to allow a --login-path=X option to read the login information so passwords did not need to be stored in clear text in a --defaults-file.