Don't create StupsTokenComponent when not needed

When the application already supplies a NakadiPublishingClient
or NakadiClient bean, we don't need our own authentication setup.

This was already meant to be skipped, but I guess we misunderstood
how nested configuration classes work – they don't inherit their
outer classes' conditions.

nakadi-producer-spring-boot-starter/src/main/java/org/zalando/nakadiproducer/NakadiProducerAutoConfiguration.java

@@ -65,6 +65,7 @@ public NakadiPublishingClient nakadiProducerPublishingClient(AccessTokenProvider
         }
 
         @ConditionalOnClass(name = "org.zalando.stups.tokens.Tokens")
+        @ConditionalOnMissingBean({NakadiPublishingClient.class, NakadiClient.class})
         @Configuration
         static class StupsTokenConfiguration {
             @Bean(destroyMethod = "stop")