save_mysql_passwd.sh needs to add a check so that it could be run only as the appdynamics user

For MySQL 5.7, save_mysql_passwd.sh creates .mylogin.cnf and the permissions on this file are 600.
So this script needs to be run as the user that is specified in db.cnf.

Many customers are running this as root, and as appd user cannot read the .mylogin.cnf the logins fail.
This file cannot be made world readable as MySQL would not accept login file that is world readable.