Fix variable typo of use_syslog in lib/tempest

TEMPEST_CONF is not set but used it. As a result the meaningless file
'DEFAULT' is created. So this commit fixes to use proper variable name.

Closes-bug: #1394969
Change-Id: If6e58f269319df3c4f71b4fc13aa855c63f7e3a3
diff --git a/lib/tempest b/lib/tempest
index d3fb9fb..399e8e6 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -247,7 +247,7 @@
         fi
     fi
 
-    iniset $TEMPEST_CONF DEFAULT use_syslog $SYSLOG
+    iniset $TEMPEST_CONFIG DEFAULT use_syslog $SYSLOG
     # Oslo
     iniset $TEMPEST_CONFIG DEFAULT lock_path $TEMPEST_STATE_PATH
     mkdir -p $TEMPEST_STATE_PATH