class mysql-server::installation {
...
      notify => Exec["Reset the root password"],
...
}

class mysql-server::service {
...
  exec {
    "Reset the root password":
      command => "/usr/bin/mysqladmin -u root password \"...\"",
      cwd => "/var/lib/mysql",
      onlyif => "/usr/bin/mysql -u root -e ''",
      refreshonly => true,
      require => Service["mysql-server"],
      user => "mysql",
  }
}