Support extra initialization file for mysql_install_db

The mysql_install_db script is used to initialize the data directory.
However, if the user needs to apply some customized init commands (for
example to change user or password) they have to start the database
with --skip-grant-tables after the install script, and then restart
the database with normal mode.

This will benefit container and distribution packaging where the provided features of mysql_install_db are insufficient.