Allow registry username to reference a secret

[kjellberg]

From DigitalOcean docs:

For CI systems that support configuring registry authentication via username and password, use a DigitalOcean API token as both the username and the password. The API token must have read/write privileges to push to your registry.

This PR enables users to configure their registry details with a secret for username, like this:

registry:
  server: registry.digitalocean.com
  username:  
    - SECRET_REGISTRY_TOKEN
  password: 
    - SECRET_REGISTRY_TOKEN

solves #89