(maint) Install PE on windows synchronously

Previously, beaker would call msiexec in a way that would cause it to
run asynchronously. This worked pretty much by chance, because beaker
did enough other stuff before trying to run puppet on the Windows
machine.

This wraps the msiexec call in `start /w`, which waits for the
installation to finish before continuing. This ensures that puppet is
fully installed before beaker continues.