Add @ConditionalOnMissingBean(MongoDbFactory)

Update MongoAutoConfiguration to be conditional on a missing
MongoDbFactory bean. The assumption is that if the user has declared
a MongoDbFactory they will either use it directly, or they will also
register a Mongo bean.

If the MongoDbFactory class cannot be found the existing Mongo
auto-configuration still applies. This ensures that users that do
not have Spring Data can still access a Mongo bean.

Fixes gh-1341

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mongo/MongoAutoConfiguration.java

@@ -27,6 +27,7 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.data.mongodb.MongoDbFactory;
 
 import com.mongodb.Mongo;
 import com.mongodb.MongoClientOptions;
@@ -41,6 +42,7 @@ import com.mongodb.MongoClientOptions;
 @Configuration
 @ConditionalOnClass(Mongo.class)
 @EnableConfigurationProperties(MongoProperties.class)
+@ConditionalOnMissingBean(MongoDbFactory.class)
 public class MongoAutoConfiguration {
 
 	@Autowired