Fix bootstrap of 64-bit 2008r2 through FreeSSHd

When connecting to FreeSSHd on 20008r2 you get a 32-bit environment. In
this environment the PROCESSOR_ARCHITECTURE is reported as i686. This
leads to the generation of an invalid URL for downloading the Chef
installer.  The workaround is to first check PROCESSOR_ARCHITEW6432 which
will show AMD64 even in a 32-bit environment.
Fix for KNIFE-285