(QENG-797) Enhance Beaker to look for x64 installers for Windows

- add ability to install 64 bit pe builds (available for pe 3.4)
- add is_x86_64? method to host, convenience method for determining arch
  of host
beaker already has logic for detecting 32 vs 64: https://github.com/puppetlabs/beaker/blob/master/lib/beaker/host/windows/pkg.rb#L43

I'm assuming `arch` is a cygwin command? in which case I think the `wmic` approach is better as it won't be confused by 32-bit cygwin running on 64-bit OS.