add(coreos-base/oem-vagrant-key): Add oem for vmware_insecure images.

vmware_insecure currently pulls in oem-vagrant which made more sense
back when that package only installed a ssh key. Now that it is oh so
much more and only works properly inside of Vagrant this new ebuild
provides a way to just get the ssh key again.
Merge pull request #575 from marineam/insecure

add(coreos-base/oem-vagrant-key): Add oem for vmware_insecure images.