Fix logging.conf sample

Use formatter from oslo_log instead of tempest.openstack.common.log
which got removed as of 583ce2c043a64778ae20a957f10cd5029e2e3230

Change-Id: I0d9334c51d50f649fdb7e34fd7c2f3d807b80601
diff --git a/etc/logging.conf.sample b/etc/logging.conf.sample
index cdeedef..36cd324 100644
--- a/etc/logging.conf.sample
+++ b/etc/logging.conf.sample
@@ -34,7 +34,7 @@
 formatter=simple
 
 [formatter_tests]
-class = tempest.openstack.common.log.ContextFormatter
+class = oslo_log.formatters.ContextFormatter
 
 [formatter_simple]
 format=%(asctime)s.%(msecs)03d %(process)d %(levelname)s: %(message)s