(PUP-390) Add Windows x64 support to MSI

 - Track the current Platform in an environment variable.  Pass it to
   candle.exe for both the -arch switch AND as the Platform variable
   used in puppet.wxi, that is part of the Package definition in
   puppet.wxs
 - Add additional variable declarations to puppet.wxi that are platform
   dependent and will change the Win64 yes / no value, the installation
   folder and some additional installer text variable as appropriate.
 - Ensure that platform dependent components from puppet.wxs have the
   win64 yes/no value set appropriate.  Since we will not support x86
   and x64 side by side installs, ensure that we always read/write
   32-bit registry values for remembering previous installer config
   values.