Bubble screensaver preferences up to node attributes

In order to make sprout-osx-settings more configurable for users, I've moved screensaver settings to attributes.
The defaults are kept the same, but now they can be overriden in a soloistrc via node attributes ^_^

Example:

    node_attributes:
        screensaver:
          ask_for_password: true
          ask_for_password_delay: 0
          timeout: 600
          disksleep: 10
          sleep: 1
          displaysleep: 2