Customizable vagrant rsync args and excludes

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'.

Vagrantfile

@@ -30,6 +30,8 @@ $kube_os = ENV['KUBERNETES_OS'] || "fedora"
 
 # Determine whether vagrant should use nfs to sync folders
 $use_nfs = ENV['KUBERNETES_VAGRANT_USE_NFS'] == 'true'
+# Determine whether vagrant should use rsync to sync folders
+$use_rsync = ENV['KUBERNETES_VAGRANT_USE_RSYNC'] == 'true'
 
 # To override the vagrant provider, use (e.g.):
 #   KUBERNETES_PROVIDER=vagrant VAGRANT_DEFAULT_PROVIDER=... .../cluster/kube-up.sh
@@ -156,6 +158,15 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
 
     if $use_nfs then
       config.vm.synced_folder ".", "/vagrant", nfs: true
+    elsif $use_rsync then
+      opts = {}
+      if ENV['KUBERNETES_VAGRANT_RSYNC_ARGS'] then
+        opts[:rsync__args] = ENV['KUBERNETES_VAGRANT_RSYNC_ARGS'].split(" ")
+      end
+      if ENV['KUBERNETES_VAGRANT_RSYNC_EXCLUDE'] then
+        opts[:rsync__exclude] = ENV['KUBERNETES_VAGRANT_RSYNC_EXCLUDE'].split(" ")
+      end
+      config.vm.synced_folder ".", "/vagrant", opts
     end
 
     # Try VMWare Fusion first (see