Set pidfile even when not running via mysqld_safe

For example, Ubuntu 14.04 /etc/init.d/mysql does not use mysqld_safe
and parses the pidfile out of `mysqld --print-defaults` instead.

templates/my.cnf.j2

@@ -8,6 +8,7 @@ port = {{ mysql_port }}
 bind-address = {{ mysql_bind_address }}
 datadir = {{ mysql_datadir }}
 socket = {{ mysql_socket }}
+pid-file = {{ mysql_pid_file }}
 
 # Logging configuration.
 {% if mysql_log_error == 'syslog' or mysql_log == 'syslog' %}