Remove non-existent config options of oslo modules

oslo.serialization and oslo.utils don't have any config options at all,
and `tempest init` command did output warning message when generating
tempest.conf. So this patch removes these entries from config-generator.

Closes-Bug: #1620861
Change-Id: Icad81bca5e3e3b69dee99fdac71174c149fd6df2

tempest/cmd/config-generator.tempest.conf

@@ -3,5 +3,3 @@ output_file = etc/tempest.conf.sample
 namespace = tempest.config
 namespace = oslo.concurrency
 namespace = oslo.log
-namespace = oslo.serialization
-namespace = oslo.utils