Only validate history topic name for affected connectors

RelationalConnectorConfig applies validation of the history topic name (see https://github.com/debezium/debezium/blob/master/debezium-core/src/main/java/io/debezium/relational/RelationalDatabaseConnectorConfig.java#L115), but this only makes sense for those connectors that actually work with a history topic, i.e. not Postgres.

Another inconsistency is that the field is validated with isRequired, but the PG docs suggest a default value and also it seems that validation isn't actually applied.

Chat discussion: https://gitter.im/debezium/dev?at=5ce3b14eb313d72314014a48