Unhardcode 'image_conversion_dir' option for controllers

Option 'image_conversion_dir' is directory used for temporary storage
during image conversion.

Change-Id: I9b72eb3f14808c68b6cff568f52e27f79d9c9cf8
Related-Prod: PROD-29437 (PROD:29437)
diff --git a/README.rst b/README.rst
index 2da3342..35a5188 100644
--- a/README.rst
+++ b/README.rst
@@ -30,6 +30,7 @@
         default_volume_type: 7k2SaS
         enable_force_upload: true
         availability_zone_fallback: True
+        image_conversion_dir: /var/tmp/cinder/conversion
         database:
           engine: mysql
           host: 127.0.0.1
@@ -260,6 +261,8 @@
   .. code-block:: yaml
 
     cinder:
+      controller:
+        image_conversion_dir: /var/tmp/cinder/conversion
       volume:
         image_conversion_dir: /var/tmp/cinder/conversion
         ...