Set root or Administrator password via admin_pass

Allow setting the admin_pass. This is especially useful on Windows since Vagrant's winrm communicator only support basic auth right now, so we need an alternative to using ssh keys.

Since the randomly generated admin_pass is never displayed or stored, pre-selecting a password via admin_pass is the only way you'll be able to connect to a windows server.

Fixing the 2nd of 3 blockers found on the windows spike (#93).