Fix Windows host file modification

 - Commit 250fad54d4851c0e853c4a38a49fb9e484f5d5f5 introduced
   using an elevated shell so that a file would be written instead of
   simply executing these commands.  Unfortunately, :elevated => true
   doesn't seem to currently work properly on Windows with Vagrant 1.6.5
   so let's revert that change.
 - The original idea behind the prior commit was to workaround script
   length limitations that may prevent all hosts from being registered
   properly, so in keeping with that spirit, refactor the PowerShell
   code to be a shorter one-liner that uses PowerShell constructs to
   remove repetitive code.