Resolves #1256, add warning mesasge to VMware ESXi driver

Resolves #1256

I added the warning message, `Please check the 'guest_os_type' in your template.json.`.
If there is no `guest_os_type` in template.json, Packer use `other` for `guest_os_type` by default.
But correct `guest_os_type` is required by `vim-cmd tools.install` command.