When using rsync file synchronization with Vagrant, the whole kubernetes
repo directory is copied over into the virtual machine. This includes
the _output directory, which tends to be gigabytes in size, while often
just _output/release-tars (a few hundred MB) would be enough.

Furthermore, if the some of the directories contains a recursive
symlink, rsync with the default args will keep descending and copying
files endlessly until filling up the VM disk.

Making the rsync args and excluded dirs/files customizable via
KUBERNETES_VAGRANT_RSYNC_ARGS and KUBERNETES_VAGRANT_RSYNC_EXLUDE,
respectively, allows the developer to prevent the issues mentioned
above. A new KUBERNETES_VAGRANT_USE_RSYNC variable is also added to
control whether Vagrant should force usage of rsync as the file
synchronization backend. The args/exclude customizations only take
effect when KUBERNETES_VAGRANT_USE_RSYNC is 'true'.