(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 ability to install 64 bit puppet builds (available for puppet 3.7+)
- install 64 bit only if machine is 64 bit and install_32 is not set for the host or globally
- add is_x86_64? method to host, convenience method for determining arch
  of host
I think we need a better (more windows-like) way of working out the appropriate install_dir rather than relying on the bash that we do here.