Set sane defaults in base VM

We should have some known sane defaults in the base VM (os_prepare role), such as:

• disabling checking for new updates
• show all files in Finder: defaults write com.apple.finder AppleShowAllFiles TRUE
• Finder, show all filename extensions AppleShowAllExtensions -bool true
• Keyboard, set a very fast keyboard repeat rate KeyRepeat -int 2 normal 6
• Keyboard, set a shorter delay until key repeat InitialKeyRepeat -int 15 normal 68