action credentials - set up user and password for an environment

This patch adds a new 'credentials' action, which sets the admin username and password for a given environment. This is  meant for environments where the roxy application will be deployed without direct shell access by developers. A system admin can unpack the application archive and run `ml credentials` to set the admin user and password to values that are not shared with the application developers. In an environment like this, using `credentials` should be less error-prone than editing a file.

After implementing this in `app_specific` I thought it might be useful for other projects.