[common] Download `.chef_databag_secret` to the appropriate location.

```
diff --git a/circle.yml b/circle.yml
index 06eec76..32e137f 100644
--- a/circle.yml
+++ b/circle.yml
@@ -5,6 +5,7 @@ machine:
 dependencies:
   post:
     - bundle exec rake init
+    - wget https://raw.githubusercontent.com/kazu634/dotfiles/master/.chef_databag_secret -O /home/ubuntu/chef/.chef_databag_secret]

 test:
   override:
```