Add missing space in enable_volume_image_dep_tests description

Change-Id: I3e2a2989c09804dfdfb78bb22a52da5f037a59bb
diff --git a/tempest/config.py b/tempest/config.py
index 36f0152..5d02238 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1122,7 +1122,7 @@
                     'types can be assigned.'),
     cfg.BoolOpt('enable_volume_image_dep_tests',
                 default=True,
-                help='Run tests for dependencies between images, volumes'
+                help='Run tests for dependencies between images, volumes '
                 'and instance snapshots')
 ]