Stacker needs to accept input parameters from the CLI.

Things like passwords that have the `NoEcho` parameters in the CloudFormation template contains sensitive information that shouldn't be hard coded in the region configuration file.

``` diff
  -DBPassword: '****'
  +DBPassword: password
  -DBUsername: '****'
  +DBUsername: username
```

A special YAML parameter should be recognized in the configuration file that would read from stdin.
