synced_folders/rsync: move rsync logic out to helper
synced_folders/rsync: `vagrant rsync` syncs the folders
synced_folders/rsync: expand host paths relative to root path
core: MixinSyncedFolders#synced_folders does scoped hash override
synced_folders/rsync: pass --delete option to delete bad files
website/docs: document `vagrant rsync`
Merge pull request #2816 from mitchellh/f-rsync-command

"vagrant rsync" non-primary command