Doc environment variables 593

fixes #593 
fixes #460 
The application.yml sets the password to clusterhq

```
(flocker-tutorial)[richard@drax flocker-tutorial]$ mysql -h172.16.255.250 -uroot -padmin
ERROR 1045 (28000): Access denied for user 'root'@'172.16.255.1' (using password: YES)
```