Add step export to esxi driver and enable to use artifacts from builder/vmware-esxi in vsphere post-processor

I guess builder/vmware-esxi should export artifacts from remote ESXi to local machine to use in post-processors.
This p-r enables following features:

* builder/vmware-esxi export specified format artifacts. (ovf, ova or vmx)
* post-processor/vsphere can use the artifacts from builder/vmware-esxi.