Unhardcode 'image_conversion_dir' option

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

Change-Id: Iec808873ecf3fbf042d5a33857fd44d23edb642b
Related-Prod: PROD-29437 (PROD:29437)
diff --git a/README.rst b/README.rst
index 6c97682..2da3342 100644
--- a/README.rst
+++ b/README.rst
@@ -86,6 +86,7 @@
         default_volume_type: 7k2SaS
         enable_force_upload: true
         my_ip: 192.168.0.254
+        image_conversion_dir: /var/tmp/cinder/conversion
         database:
           engine: mysql
           host: 127.0.0.1
@@ -254,6 +255,15 @@
         wipe_method: shred
         ...
 
+* Configure directory used for temporary storage during image conversion:
+
+  .. code-block:: yaml
+
+    cinder:
+      volume:
+        image_conversion_dir: /var/tmp/cinder/conversion
+        ...
+
 * Configuration of ``policy.json`` file:
 
   .. code-block:: yaml