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.
Yes, `-ssh-key-name=` needs to be fixed (last I knew it didn't work because I observed coretest clobbering keys) and a reasonable name should be derived from the source or the units need to be modified to pass that option. adding key name to the current units is awkward since most are a common template that just gets the path to the cloud config as an argument.

Also, I've always wanted `-ssh-key-name=` to be more generic so it could alter the default name of dropin files and what not. The more we can keep different config sources from clobbering each other the better.