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/tests/pillar/control_single.sls b/tests/pillar/control_single.sls
index 9f6d66d..61b7386 100644
--- a/tests/pillar/control_single.sls
+++ b/tests/pillar/control_single.sls
@@ -3,6 +3,7 @@
     enabled: true
     version: liberty
     scheduler_default_filters: AvailabilityZoneFilter
+    image_conversion_dir: /var/tmp/cinder/conversion
     osapi:
       host: 127.0.0.1
     database: