provisioners/ansible: Replace inventory file instead of modify

When updating the inventory, write to a temp file and replace the
original once writing is complete, to allow for an atomic replacement
of the contents.

Ensures that ansible reading an inventory file will get either the old
or new contents, but never the truncated version of the file that
appears should you open it with 'w' mode set to replace the contents.

Solves the 'provided hosts list is empty' error, which is emitted by
ansible should it manage to be reading the inventory file just as it
was truncated, but before the new contents were flushed to disk.

Partially-Fixes: #6526

plugins/provisioners/ansible/provisioner/host.rb

@@ -132,8 +132,15 @@ module VagrantPlugins
           inventory_file = Pathname.new(File.join(inventory_path, 'vagrant_ansible_inventory'))
           @@lock.synchronize do
             if !File.exists?(inventory_file) or inventory_content != File.read(inventory_file)
-              inventory_file.open('w') do |file|
+              begin
-                file.write(inventory_content)
+                # ansible dir inventory will ignore files starting with '.'
+                inventory_tmpfile = Tempfile.new('.vagrant_ansible_inventory', inventory_path)
+                inventory_tmpfile.write(inventory_content)
+                inventory_tmpfile.close
+                File.rename(inventory_tmpfile.path, inventory_file)
+              ensure
+                inventory_tmpfile.close
+                inventory_tmpfile.unlink
               end
             end
           end