add open-vm-tools

This PR is an attempt at fixing #499. It was a quick hack, and probably could use some cleanup, but I'd like to get feedback regarding the approach. In particular, I'm not very happy that it would end up in every image, even non-vmware ones. But I'm not sure there's an easy way to avoid that, or even if it matters at all.

This adds a customized version of open-vm-tools to the CoreOS build
- dependency to open-vm-tools-kmod is removed to avoid out-of-tree
  kernel modules
- pic is enforced to avoid link error
- hgfsmounter is forcively disabled to avoid binary conflict between
  /usr/bin and /bin (plus it's useless without the corresponding
  kernel module anyway)
- dnet dependency is removed as not part of CoreOS for now

The purpose of this package is primarily to make the vmtoolsd service
available, which allows communication back and forth between the VM
and the host. In particular this allows:
- to control the memory balloon
- to implement correctly power operations