Remove tmpfs mount on /tmp and bring back /tmp cleanup on halt

fixes #406 because the `tmpfs` mount isn't needed anymore.

Careful considerations had to be taken because #68 mentions host-side
data loss when cleaning up `/tmp`. We mitigate this by ensuring that all
mounts under `/tmp` are unmounted before we proceed with our `rm -rf`
operation.

More context about this issue can be found in #360.

lib/vagrant-lxc/action.rb

@@ -12,6 +12,7 @@ require 'vagrant-lxc/action/handle_box_metadata'
 require 'vagrant-lxc/action/prepare_nfs_settings'
 require 'vagrant-lxc/action/prepare_nfs_valid_ids'
 require 'vagrant-lxc/action/private_networks'
+require 'vagrant-lxc/action/remove_temporary_files'
 require 'vagrant-lxc/action/setup_package_files'
 require 'vagrant-lxc/action/warn_networks'
 
@@ -124,6 +125,7 @@ module Vagrant
             end
 
             b2.use ClearForwardedPorts
+            b2.use RemoveTemporaryFiles
             b2.use GcPrivateNetworkBridges
             b2.use Builtin::Call, Builtin::GracefulHalt, :stopped, :running do |env2, b3|
               if !env2[:result]

lib/vagrant-lxc/action/boot.rb

@@ -26,9 +26,6 @@ module Vagrant
             config.customize 'mount.entry', '/sys/fs/selinux sys/fs/selinux none bind,ro 0 0'
           end
 
-          # Make /tmp a tmpfs to prevent init scripts from nuking synced folders mounted in here
-          config.customize 'mount.entry', 'tmpfs tmp tmpfs nodev,nosuid,size=2G 0 0'
-
           env[:ui].info I18n.t("vagrant_lxc.messages.starting")
           env[:machine].provider.driver.start(config.customizations)
 

lib/vagrant-lxc/action/remove_temporary_files.rb

@@ -0,0 +1,23 @@
+module Vagrant
+  module LXC
+    module Action
+      class RemoveTemporaryFiles
+        def initialize(app, env)
+          @app    = app
+          @logger = Log4r::Logger.new("vagrant::lxc::action::remove_tmp_files")
+        end
+
+        def call(env)
+          @logger.debug 'Removing temporary files'
+          driver = env[:machine].provider.driver
+          # To prevent host-side data loss, it's important that all mounts under /tmp are unmounted
+          # before we proceed with the `rm -rf` operation. See #68 and #360.
+          driver.attach("findmnt -R /tmp -o TARGET --list --noheadings | xargs -L 1 --no-run-if-empty umount")
+          driver.attach("rm -rf /tmp/*")
+
+          @app.call env
+        end
+      end
+    end
+  end
+end