Handle box url from Vagrantfile

Closes #17

example/Vagrantfile

@@ -10,6 +10,7 @@ Vagrant.configure("2") do |config|
   config.vm.synced_folder "/tmp", "/vagrant_data"
 
   config.vm.provider :lxc do |lxc|
+    config.vm.box_url = '../boxes/output/ubuntu-cloud.box'
     lxc.start_opts << 'lxc.cgroup.memory.limit_in_bytes=400M'
     lxc.start_opts << 'lxc.cgroup.memory.memsw.limit_in_bytes=500M'
   end

lib/vagrant-lxc/action.rb

@@ -106,6 +106,7 @@ module Vagrant
           b.use Vagrant::Action::Builtin::Call, Created do |env, b2|
             # If the VM is NOT created yet, then do the setup steps
             if !env[:result]
+              b2.use Vagrant::Action::Builtin::HandleBoxUrl
               b2.use HandleBoxMetadata
               b2.use Create
             end