instead of duplicating vsphere stemcell building stages to `image_vcloud_ovf`, `image_vcloud_prepare_stemcell`, `image_vcloud_vmx` please make existing vsphere stages more generic like so:

Stages:
- `image_vsphere_ovf` becomes `image_ovf_generate`
- `image_vsphere_prepare_stemcell` becomes `image_ovf_prepare_stemcell`
- `image_vsphere_vmx` becomes `image_ovf_vmx`

Variables:
- `$work/vsphere` becomes `$work/ovf`
- `image_vsphere_ovf_ovftool_path` becomes `image_ovftool_path`

After doing that the stemcell builder runner which lists which stages to run needs to be adjusted to use new stages.