setmysqlpass: be more careful before plundering into action

Since this script is rooting the database, it'd be good to use a little
more precaution so that we don't let systems be in an inconsistent case
when crashing.

In cases where the PATH variable is not appropriately set (variable is
empty by default when script is invoked by puppet) the script shuts down
mysql and then is not able to call most commands.