Add config.cache.root option to specify a custom cache root.

Only available for single box environments.
Requires that the custom cache root already exists - because it is
custom, it is your responsbility instead of vagrant-cachier's.
Fixes #87.

lib/vagrant-cachier/action/configure_bucket_root.rb

@@ -21,7 +21,19 @@ module VagrantPlugins
         end
 
         def setup_buckets_folder
-          FileUtils.mkdir_p(cache_root.to_s) unless cache_root.exist?
+          custom_root = @env[:machine].config.cache.root
+          if custom_root != nil
+            case @env[:machine].config.cache.scope.to_sym
+              when :box
+                unless File.directory? custom_root
+                  raise "Custom cache root must already exist and be a directory"
+                end
+              else
+                raise "Custom cache root only supported in single box environments"
+            end
+          else
+            FileUtils.mkdir_p(cache_root.to_s) unless cache_root.exist?
+          end
 
           synced_folder_opts = {id: "vagrant-cache"}
           synced_folder_opts.merge!(@env[:machine].config.cache.synced_folder_opts || {})
@@ -33,6 +45,8 @@ module VagrantPlugins
         def cache_root
           @cache_root ||= case @env[:machine].config.cache.scope.to_sym
             when :box
+              custom_root = @env[:machine].config.cache.root
+              return custom_root unless custom_root == nil
               @box_name = box_name
               # Box is optional with docker provider
               if @box_name.nil? && @env[:machine].provider_name.to_sym == :docker

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, :root
       attr_reader   :buckets
 
       ALLOWED_SCOPES = %w( box machine )
@@ -11,6 +11,7 @@ module VagrantPlugins
         @auto_detect = UNSET_VALUE
         @synced_folder_opts = UNSET_VALUE
         @ui = Vagrant::UI::Colored.new
+        @root = UNSET_VALUE
       end
 
       def enable(bucket, opts = {})
@@ -51,6 +52,7 @@ module VagrantPlugins
         @auto_detect = true if @auto_detect == UNSET_VALUE
         @synced_folder_opts = nil if @synced_folder_opts == UNSET_VALUE
         @buckets = @buckets ? @buckets.dup : {}
+        @root = nil if @root == UNSET_VALUE
       end
 
       private