Make shelved_offload_time's help message more specific

This is to make shelved_offload_time's help message explicitly
mention "[nova.DEFAULT]->shelved_offload_time in nova.conf".

Change-Id: I0c7feb13de983516c126171ff1457392eff0135e
diff --git a/tempest/config.py b/tempest/config.py
index b14d4fd..5eee86d 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -312,9 +312,9 @@
                default=0,
                help='Time in seconds before a shelved instance is eligible '
                     'for removing from a host.  -1 never offload, 0 offload '
-                    'when shelved. This time should be the same as the time '
-                    'of nova.conf, and some tests will run for as long as the '
-                    'time.'),
+                    'when shelved. This configuration value should be same as '
+                    '[nova.DEFAULT]->shelved_offload_time in nova.conf, and '
+                    'some tests will run for as long as the time.'),
     cfg.IntOpt('min_compute_nodes',
                default=1,
                help=('The minimum number of compute nodes expected. This will '