add open-vmdk 1.0 (utility to create vmware ova files)

I moved the ebuild file to coreos-overlay, so this replaces https://github.com/coreos/portage-stable/pull/157.

This PR is needed for https://github.com/coreos/scripts/pull/364. It adds open-vmdk. This is a set of tools to create a stream-optimized vmdk, and an ova file from that, to be installed in VMware ESXi, Workstation or Fusion.

I don't know where to add this as a dependency to the SDK, so that it gets built and installed in the SDK by default. Help is appreciated.