Adds open-vm-tools, libdnet and oem-vmware packages

This PR continues on the work started by @crawford on porting open-vm-tools. Additionally, it adds the oem-vmware ebuild and a coreos-init change to add a SystemD network unit in order to enable receiving DHCP offers through broadcast, for VMware images **only**.

Except for the systemd network unit added in coreos-init, these three packages and their dependencies are only installed on VMware OEM images.

This PR is part of a larger change-set intended to create a good starting point for VMware OEM images. The change-set is composed of the following PRs: https://github.com/coreos/scripts/pull/336, https://github.com/coreos/coreos-overlay/pull/898, https://github.com/coreos/init/pull/137, https://github.com/coreos/portage-stable/pull/139
Due to the odd way OEM packages are installed they cannot define dependencies on packages from the base system. A comment noting what is expected of the base system will have to do.