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/tests/pillar/volume_single.sls b/tests/pillar/volume_single.sls
index e81a2cd..c1fd5ec 100644
--- a/tests/pillar/volume_single.sls
+++ b/tests/pillar/volume_single.sls
@@ -4,6 +4,7 @@
     version: liberty
     scheduler_default_filters: AvailabilityZoneFilter
     default_volume_type: defaultVolumeType
+    image_conversion_dir: /var/tmp/cinder/conversion
     backend:
       lvm:
         host: 127.0.0.1