www@toolbox:~/shift/ui/config/environments$ cat development.rb
Rails.application.configure do

... snipped ...

  ## mysql_helper
  # read-only credentials for shift to connect to and inspect all hosts
  config.x.mysql_helper.db_config = {
    :username => "shift",
    :password => "xxx",
    :host => "192.168.x.y"
  }

... snipped ...

end
www@toolbox:~/shift/ui/config/environments$ mysql -u shift -p -h 192.168.x.y
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2441077
Server version: 5.6.30-76.3 Percona Server (GPL), Release 76.3, Revision 3850db5

Copyright (c) 2000, 2016, 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> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| redacted           |
| mysql              |
| performance_schema |
| shift_demo         |
| shift_development  |
| shift_test         |
| sys                |
+--------------------+
8 rows in set (0.00 sec)

mysql> select * from clusters\G
*************************** 1. row ***************************
                   id: 1
                 name: default-cluster-001
                  app: local-app
              rw_host: 192.168.x.y
                 port: 3306
admin_review_required: 0
           is_staging: 1
1 row in set (0.00 sec)

mysql> exit