builder/vmware-esxi: Add step_export

If `format` option is configured, packer exports the VM with ovftool.

website: Document about OVF Tool and `format` option.

post-processor/vsphere: Enable to use `mitchellh.vmware-esx` artifact type and OVF and OVA formats, fixes hashicorp#1457.

builder/vmware-esxi: Add step_export

If `format` option is configured, packer exports the VM with ovftool.

website: Document about OVF Tool and `format` option.

post-processor/vsphere: Enable to use `mitchellh.vmware-esx` artifact type and OVF and OVA formats, fixes hashicorp#1457.

Fixes hashicorp#1457, post-processor/vsphere: enable to use `mitchell…

…h.vmware-esx` artifact type and OVF and OVA formats.

`resource_pool` option become optional.

builder/vmware-esxi: Add step_export

If  option is configured, packer exports the VM to output directory with ovftool.

website: Document vmware-esxi OVF Tool requirements, and 'format' option.

builder/vmware-esxi: Add step_export

post-processor/vsphere: Enable to use artifacts from builder/vmware-esxi
website: Document vmware-esxi OVF Tool requirements, and 'format' option

Merge pull request hashicorp#1964 from lyrixx/better-errror

Better error reporting when a config key in template is Unknown

builder/vmware-esxi: Add step_export

post-processor/vsphere: Enable to use artifacts from builder/vmware-esxi
website: Document vmware-esxi OVF Tool requirements, and 'format' option