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.

nit, we try to keep to Gentoo's recommended style of using tab characters in ebuilds.