Extend user module, add 'disable user'

This patch adds the user::virtual::disable function to the user module.
This will allow puppet to remove a user, ssh keys, and screen sessions
while preserving the user home directory.

This patch adds future functionality without impacting the currently
configured infrastructure.

Change-Id: I2933e6857094398f86c2a7e6eaabe9898a1d3078
Merge "Extend user module, add 'disable user'"