disable hgfs, add rsync option for shared folders, note core sshkey conf...

When using a coreos-vagrant box with the vmware_fusion provider, to host containers using the docker provider, hgfs must be disabled.

I also added a comment about using rsync for shared folders, and a note/warning about setting the ssh_authorized_keys in user-data as this overrides the oem settings and breaks things in interesting ways.
@marineam is this a bug? Shouldn't different cloud-init invocations be additive for ssh keys?