Use empty string for mysql root user password

[yahonda]

so that users can login to mysql without typing password.

Addresses #122

vagrant@rails-dev-box:~$ mysql -uroot
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.20-0ubuntu0.17.10.1 (Ubuntu)

Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

[yahonda]

Closing it since it does not match the comment.

rails-dev-box/bootstrap.sh

Line 47 in 7cb1f29

# Set the password in an environment variable to avoid the warning issued if set with `-p`.