First shot at adding a config option to override the directory used on the host machine

lib/vagrant-cachier/action/configure_bucket_root.rb

@@ -21,12 +21,13 @@ module VagrantPlugins
         end
 
         def setup_buckets_folder
-          FileUtils.mkdir_p(cache_root.to_s) unless cache_root.exist?
+#         FileUtils.mkdir_p(cache_root.to_s) unless cache_root.exist?
 
           synced_folder_opts = {id: "vagrant-cache"}
           synced_folder_opts.merge!(@env[:machine].config.cache.synced_folder_opts || {})
+          puts(@env[:machine].config.cache.override_directory)
 
-          @env[:machine].config.vm.synced_folder cache_root, '/tmp/vagrant-cache', synced_folder_opts
+          @env[:machine].config.vm.synced_folder @env[:machine].config.cache.override_directory || cache_root, '/tmp/vagrant-cache', synced_folder_opts
           @env[:cache_dirs] = []
         end
 

lib/vagrant-cachier/config.rb

@@ -1,7 +1,7 @@
 module VagrantPlugins
   module Cachier
     class Config < Vagrant.plugin(2, :config)
-      attr_accessor :scope, :auto_detect, :synced_folder_opts
+      attr_accessor :scope, :auto_detect, :synced_folder_opts, :override_directory
       attr_reader   :buckets
 
       ALLOWED_SCOPES = %w( box machine )